This is actually my friend's problem, not mine. But I have definitely decided to put off upgrading until this problem is addressed.
He is a relatively new S1 user, but a very experienced producer on other platforms. Loves S1 4.5, bought it on my (indirect) recommendation. Just upgraded to 5. Now the same song that barely taxed his computer at all in 4.5 uses so much computer power that it glitches, even with high buffer settings. He did another similar experiment, with two identcal songs. 4.5 works fine. 5 glitches.
Sorry for the incomplete info. I can get more details. I am asking mostly because I was considering upgrading but will now hold off until I get some reassurance that this will not happen to me as well.