Windows 10 was a HUGE HUGE performance upgrade from Win 7 in EUIV for me; I haven't measured it but the months on speed 5 seem to go almost twice as fast. (Not exaggerating thats what it feels like) the game became a LOT smoother and faster. I feel like my i7 is finally using its true power.
On the other hand..... CK2 runs half as fast (or even less tbh), the months go sluggishly, old late-game saves are almost unplayable. And plenty of 'laggy' bugs, like when I click on a province it takes like a second or so for it to register on speed 5 (used to be instantenous). Sometimes the clicks don't register at all (on any speed even) which necessiates clicking more than once (this can be fixed by alt-tabbing and then back again for me at least). Playing on speed 5 is a lot less enjoyable in CK2 that's certain.
Although I am unsure if these performance problems are because of WIndows 10 or the Horse lords patch. (prob mixture of both).
I haven't noticed a difference in HoI3, though if anything it plays slightly faster.
Well definitely my EUIV performance improved greatly - that is literally my main reason for not reverting to Windows 7.
EDIT my hardware - i7-4790, GTX 750 ti, 8 GB RAM, 64 bit windows 10 & a standard hard drive.