Whoa, talk about a sleeper hit out of bloody nowhere; This game is AMAZING. It's everything I ever wanted a HOMM game to be and more, cranked up to 11. The difficulty is also borderline unbearably brutal at even medium levels.
Regarding the performance issues, yeah, they're pretty bad, especially on Win 7 64bit, which I'm currently running. There are several solutions, one is to decrease cpu usage as the previous commenter said. Another, is to free up resources by turning off even the most miniscule of programs; steam, internet browser and (maybe, depends on your system) even the firewall.
Both of these solutions worked for me, but miraculously enough the game ran absolutely fine two days in a row without jumping through any aforementioned hoops.
As for the wizard, the only way I found him to be useful as a starter hero, is to spend energy at the start of an invasion to boost your crystal income, otherwise it's hopeless, you'll run out of them much too soon. And even then, he is a glorified scout with equal damage output, but less troops and less hitpoints; everyone always beelines to slay him every single time. Until you get those high level spells that is, and that's 1.) rare as hell to find, 2.) you'll probably get something useful by the middle-end of the game, and by that point you would have been much better off starting out with say, a warrior instead.
Last but by far not least, it all depends on the difficulty. Your tactics, availability of resources and overall strategy will widely differ between say, competent and expert. What may be hilariously efficient on one difficulty level will get you killed on another, the beat and rhythm of the game is widely different and always requires massive readjustments in how you play, so I'd start the game already on a difficult but fair level, which is expert, as it gives no bonuses or penalties to the ai or you, and learn from there.
Again I repeat, I don't think even Heroes of Might and Magic has grasped my attention this much back in the day, this game is glorious, and what's even more difficult to comprehend is it's all the work of just one imaginative russky.