This weekend I fired up the Yuri's Revenge campaign for a spell, and again I meet my mortal enemy: really fast gamespeed.
Not to say I can't handle it, but anybody know how to slow down singleplayer? Editing the .ini file directly changes nothing.

Whenever you go to start a single player game, there is a tab at the bottom right saying "Game Speed". Put that down.
That is, if my memory serves correctly.
If that fails, if you have some hardware from AMD, get a special add-on from them, it corrects speed issues (I remember the game speed issue I had with KOTOR, when I got the AMD add-on it went away).

There are also free programs that reduces the CPU speed allotted for a program.

run some Folding@Home, trying to brought down its speed.

And use task manager to low down its priority level.

What DH said. When the game starts the speed is automatically set to fastest, you'll need to adjust it.

Speaking of YR, I've dl the Mental Omega mod. Anyone have any ideas on how to defeat the massive AI hordes that comes at you very soon into a skirmish game? It's especially difficult playing against the allies, with barely one or two defences up and facing ten or twenty prism tanks.

There are also free programs that reduces the CPU speed allotted for a program.

DOSbox has that option as well.