@michelleealexis The best mod to control strays, and perhaps the only one, is NRaas Register. There are a number of NRaas mods that help with performance in various ways; if you're interested, I can give you a list and a short explanation of each. Or you could ask at NRaas directly. The people there are friendly and helpful and would be happy to discuss which mods will provide the features you want in your game.
Still though, it would be better to start with no mods, and no cc (yet), and test in Appaloosa Plains. Then you can add mods if you want and see whether it helps. It sounds like your custom world may well have routing issues, which would explain at least some of the lag. The point of testing in AP or Sunset Valley is to know how your game runs at its best, so you'll be able to tell if a new world, or an older save, is falling off in performance, and do something about it sooner rather than later.
When hwinfo is running, it keeps track of the current, minimum, maximum, and average values for all the fields it tracks, including temperature, for the duration of the time it's been running. So even after you quit TS3 and the heat starts to dissipate, you'll still be able to see what the max temp was for various components.
There are a few different approaches you can take, but the way I'd go is to play in fullscreen if that's what you normally do, and then pause the game every ten minutes, switch to windowed mode in graphics options (you can do this on the fly, it's fine), and check the temperature readings. Then back to fullscreen for another ten minutes. This may be tedious, but it's only to establish a baseline, to see how your computer performs. If everything looks fine, you can set alarms in hwinfo, open it before you start playing, and then completely ignore it. (I mean, be sure to check it after you quit, just so you have an idea of what's going on, but no need to monitor while you play.) The point of setting an alarm is that you don't need to monitor yourself, you'll get the alert if your temps rise too high.
@puzzlezaddict Okay epic! Will the alarm go off when I Olay in fullscreen mode? Also if you want to, I would LOVE a list of helpful mods and a quick explanation.
@michelleealexis The alarm will definitely go off; the question is whether you'd see it. You might have to allow notifications; I'm not sure. But you could easily experiment, setting an alarm for a low number that you know your system will exceed quickly. If not, there's always Windowed Borderless Gaming, which works very well with TS3.
Or you could just figure out how hot your computer runs normally. Once you've set a baseline, you might realize you're fine switching to windowed mode once an hour to check and otherwise leaving it alone. The only way to find out is to run the tool and see what kind of temperatures you're getting.
The mods I use to keep my game running smoothly are all from NRaas, which has very high standards and a large user base to test before anything gets released. Here's a list to start with:
There are plenty more mods at NRaas, but it's usually better to figure out what function you want in-game, then find the mod to fit, rather than keep adding them to see what they do. Besides, this seems like a long enough list to start with. Once you're comfortable with these, and your game is running well, you can start expanding.