For the most part, your OS will probably govern the speed and temperature of your CPU. You can do certain things to point your OS in the right direction like ni

For the most part, your OS will probably govern the speed and temperature of your CPU. You can do certain things to point your OS in the right direction like nicing your program at a low value, but I don't think there's a "limit thread frequency" value or anything like that you're missing. Edit: just read that this is about MacOS. IIRC, GCD will move certain programs to exclusively use your efficiency cores if you set their priority low enough. I forget how you do that exactly though, so you're kinda on your own.