This AI behaviour would be merely eccentric if it didn’t cause major performance issues with the game.

For the case of too many science ships, the effect seems subtle – just a small, evenly-spread slow-down of the game. Probably not worth worrying about generally, though if you happen to notice an AI empire spamming science ships, it may be worth addressing.

For the case of too many construction ships, the effect is much more pronounced. Every AI construction ship triggers an expensive script periodically, and all aligned together in time, which can result in noticeable hangs every seven in-game days if there’s many of them.

And “too many” is a surprisingly small number – as few as ten can cause five-second-long hangs every seven in-game days, in my experience. I suspect their detrimental effect is superlinear to their number (because the AI having only a couple seems to incur no noticeable overhead). It’s likely also contingent on some other factor, that I have not yet identified, as the hangs only manifest themselves in some games.

I’m very thankful to Confident_Chance_770 for pinning the periodic hangs down to this cause.

Confirming the too-many-construction-ships diagnosis

Since this can be caused by a mere few construction ships – as little as four or five, which is not otherwise remarkable – one way to confirm this is the issue is to profile the game’s script execution:

1. Pause the game (e.g. hit the spacebar). Ideally a day or so before the next hang day, and clear of the first day of the in-game month.
2. Hit the backtick (`) key (top-left on most keyboard layouts, above tab and left of 1) to bring up the game’s debugger / cheat console.
3. Type “script_profiler” and hit return.
4. Unpause the game (e.g. close the console by hitting the backtick key again, then hit the spacebar).
5. Let the game run until one of the hang days is encountered. Note that the game will run slower than normal while profiling is enabled, especially on the hang days.
6. Pause the game.
7. Open the console again.
8. Type “script_profiler” again and hit return. This concludes the profiling and dumps the results to two files.
9. Open the summary results from ~/Documents/Paradox Interactive/Stellaris/logs/script_profiling_summary.log.
10. Check if “planet_possible.find_planet” is the topmost item and listed as consuming significant amounts of CPU time (e.g. 5s, for a single hang day). If it is, you’re likely experiencing the construction ship bug.
    
    If it’s not, it’s possible it’s still the same bug, but possibly not. You could try the usual diagnostic routine – e.g. websearch for the symptoms and top script names from the profile, contact the author of any suspicious or newly-added plug-ins you’re using, contact Paradox Interactive, etc.

Remedying the bugs

If you determine that it likely is the construction / science ship spam bug, you can rectify it via these steps:

1. Save your game. Just in case something goes wrong with this process.
2. Pause the game. This is very important – if you don’t, the AI is likely to complete screw with your empire while you’re away (during steps 4 through 6).
3. Open the console.
4. Type “play N” where N is an integer starting at one. Not all integers are used – just keep incrementing until you get a correct result, which is that it switches your control to one of the AI empires.
5. Close the console.
6. Check the civilian ships list to see if there are absurd numbers of science ships, or more than a couple of construction ships. If there are, kill them. Usually when this bug occurs most of the ships are together in one system, so you can accelerate this process by finding those big group(s) of construction & science ships, drag-selecting them within the system view, then hitting the Delete key (and return to confirm decommissioning). It’s pretty safe to just delete all the AI empire’s construction & science ships, since the AI can cheaply and quickly build new ones if they actually want them (in my experience they often don’t, at least not promptly, suggesting the ships were unnecessary to begin with).
   
   If there are not, go back to step 4 and try subsequent integers, to check other AI empires.
7. Once you’ve found and destroyed the offending ships, type “play 0” into the console to restore control to your own empire.
8. Unpause the game and continue playing – hopefully now without the noticeable hangs every seven in-game days.

Repeat as necessary when the problem recurs (and perhaps focus your in-game efforts on wiping out the offending AIs, as a more permanent solution).

As best I can tell it’s fatally buggy, because it lets the AI essentially cheat, and you can’t turn that misfeature off.

But in its defense it kind of sets expectations right from the beginning. My very first game failed as described above, though it was at least because my empire was weak and I’d made some actual mistakes. There’s no manual or tutorial, just the in-game tips which are sparse, terse, and simply don’t cover a lot of essential elements of gameplay. So my first abrupt loss was unsurprising – unfortunately, trial and error is the name of the game here.

Subsequent games went no better, however.

I eventually found that there’s a config option when starting a new game, for how many AIs start way ahead of you. Ah, that sounds like the problem. Unfortunately even when set to zero, at least one AI still does this. And it’s not just a little ahead – they seem to start completely advanced in tech tree and with effectively infinite resources. And the most infuriating of all is that they can lurk for hours of gameplay before actually deciding to annihilate you on a whim. I’d appreciate if they just wipe me out straight away and save me all that wasted time.

For example, in my most recent game I had taken control of nearly a third of the galaxy, beating out half a dozen other AIs, and was shown by the game as equal or greater in strength than all the AIs I’d contacted. And then an AI from the other side of the galaxy declared war on me without any warning – I didn’t even know they existed – and wiped me out in literally about sixty seconds. They cleared out all my space stations, all my defenses, killed my main battle fleet in literally a single shot, and made short work of taking over all my planets with hundreds of invading armies that individually were more powerful than an entire planet’s defences. They had multiple battle fleets that were each more than a hundred times more powerful than my entire armada.

So that sucks. I went back through my saves to try to find a branch point where I could retry from, but came to the sad conclusion that there’s no possible way to beat that AI – at best I can hope it simply doesn’t decide to kill me. But they always do, eventually.

And it’s depressingly insightful what happens next, which is to say, nothing… the game just keeps going, with you unable to do anything at all – all your planets are conquered, all your shipyards long gone, all your ships at best hiding in dark corners of the galaxy. And yet the game just keeps trudging along, revealing the truth: it’s not for you. You were just a transient.

For the sake of argument, though, let’s hypothesise that they soon fix this crippling bug. In that case, the game is… meh. There’s a lot of time spent waiting for something to happen. Particularly at the beginning, when your empire is small and you’re completely growth & resource constrained, it’s 90% just waiting for the real game to develop. The ‘Fastest’ game speed option isn’t nearly fast enough.

It shares a lot with Cities: Skylines, in that respect. A pretty, intricately detailed game that invites you in alluringly, but turns out to be fundamentally repetitive, and missing the actual gameplay part. Not to mention the fun that traditionally accompanies games.

One more point, because it’s something I always look and hope for particularly in strategy games like this – deep diplomacy. Unfortunately, in Stellaris the ‘diplomacy’ aspects are pointless. Aside from the options available being rudimentary and very difficult to manage – good GUIs for diplomacy have been well established by many other games, so it’s a mystery why they couldn’t follow suit – it’s moot in any case though, as AIs will never, ever give you anything without you giving them way more in return. Most AIs won’t even deal with you. And there’s also crippling bugs in the diplomacy code too, such as the fun one where you finally find a deal the AI might accept, that’s not totally, ubsurdly unfair, and you offer it, and then the game enacts all the things you promised to give to the AI and not one single thing they were required to contribute.

Suffice to say Stellaris is a pretty big waste of money, and more importantly, time.

In many respects it feels heavily ‘inspired’ by Endless Space. Endless Space can be unforgiving to beginners, and has a few minor bugs & GUI flaws, but it’s actually winnable if you give it some time and develop some skill, and can be enjoyable to play if you invest some time to learn it. So just go play that instead, and save yourself money & irritation.