Okay, this is likely going to take a couple of back-and-forth questions here, but one thing I'll point you to is this, which has some good stuff even for single player:
http://www.arcengames.com/forums/index.php/topic,74.0.html+10 SpeedFirst of all, regarding the "bug" with +10 speed, that's not actually a bug, that's just the way the processor gets prioritized by the unity engine when you run it at a faster sim rate than it can handle. I would expect for +10 speed to be unplayable on most computers, honestly, so the fact that you've just got a bit of mouse lag is a surprise to me. The reason that +10 exists at all is for people with overclocked power computers, and for those who want to "fast forward" briefly and then slow it back down to something they can play at.
If you're just wanting a generally faster game, then there are better ways of going about it than playing at +10 speed all the time (the higher speed settings in general, even +1, aren't meant to be played on for extended lengths of time, even though you can -- it linearly increases the load on your CPU). At any rate, it sounds like you might actually prefer a different combat style, which you can choose in the lobby. Blitz makes ships move much faster, and thus leads to a faster game. If you're finding that resources are coming in too slowly for your playstyle (which is unusual, but happens), then you can also give yourself and both AIs an equal positive handicap. As long as you and the AIs have the same handicap, the game doesn't consider it cheating, and everything (achievements, etc) will happen like normal.
Both of these are CPU-neutral ways of increasing the speed of the flow of the game, as opposed to running vastly more sim cycles per second, which will eat at even a mega processor.
Only one processor core being peggedTHAT isn't natural, unless your not the host in a multiplayer game. But if you're playing solo or you're the multiplayer host, you should have two processors in heavy use. One for the main game sim, and one for the AI thread. However, what's most likely going on is one of two things:
1. The AI thread just doesn't have to think about that much when you happen to look at it, so it's usage is not as high as the main sim that you're running at +10.
2. The Windows Task Manager is known for being really poor about saying which cores are actually in use; it tends to either average things out between multiple cores when really it's all on one, or combine them into one when really it's on several. I don't know why that is.
General late-game performance woes (if it's CPU)That is really a surprise, unless you're playing at a higher speed than baseline. If you're trying to do battles at +10, or even +1, then I'd expect your performance to be absolutely in the gutter. As I noted, that speed increase feature is a linear burden on the CPU, and the battles are geometrically more of a burden than the normal game sim.
If you turn the speed of the game down to -10, that's running in slow-mo and it's going to run fewer calculations, but it also makes for more input lag so it will feel laggier even if it's not -- it turns
everything down, including input polling. So that could account for that, I'm not sure.
However, if you're running on a really low performance profile and still having problems, AND you're running into this problem on many games, then that makes me think that this isn't a CPU problem at all, but rather a GPU one.
General late-game performance woes (if it's GPU)What sort of resolution are you running the game at? If you're running this on Eyefinity or something, and your GPU can't handle that, then that could be the problem right there. One way to test this is to get into a big battle that is causing lots of slowdown for you, and then just scroll out to empty space and look at nothing. How is that affecting your performance? Is everything suddenly great? If so, then probably you want to try running the game at a lower screen resolution, and see if that helps. Personally I'm running an 8800GTS and never seem to have any substantial dips, but traditionally I've been running at 1280x1024.
It's also always possible that you've got a buggy graphics card driver -- it happens. You might try upgrading to the latest one, or if you're already on the latest one you might see if anyone else has been having any trouble in any other games, and what they downgraded to if there is trouble. A year and a half ago there was a specific run of nVidia drivers that was causing problems for people in a wide variety of games, and it took them several versions to work through that; that doesn't happen often, but it does happen. I definitely hope that's not what this is, though, as that's a major pain for everyone (you won't be the last with this problem, if that's what it is).
The other possibility: some sort of in-game bottleneckIt's possible that there's some sort of specific inefficiency in the scenario that you're playing. That happens from time to time, where players discover some edge case where there's 3k dyson gatlings roaming around or whatever, and that just eats their CPU. If you have a savegame that is demonstrating your problem that you can post, then we can take a look and see what we can see. I'm not aware of any such problems in 5.0 or 5.011, but it's a big game and you could be the first to discover something obscure. The reason I discount this possibility is that you say you've started many different games that all exhibit this problem, and that makes the likelihood of some rare confluence of ships and gamestate go way down. Unless you're always playing the same unusual scenario or something, but I don't know that that seems too likely.
Anyway, those are my thoughts for now -- hope that helps, and let me know what you find out further. Thanks!