Okay the problem was threefold.
The Spectre fix may have indeed broken the camel's back. But it was the least of my concerns, so read on, dear friend, read on (I'm actually glad about it in hindsight because it pointed me in the right direction)
I thought, the current weather around here doesn't really help my CPU load either. Must make things worse. But wait, why isn't my cpu fan screaming from below the desk if Reason's struggling so hard...? More CPU load produces heat, right?
Unfortunately I can't tell you the last and most important part of the story because it's super embarassing
Oh okay, since you asked so nicely and polite and always treat me well -
Checked my system with a sensor monitoring software. Long story short:
CPU fan wasn't plugged in, already forgot I cleaned that fucker a few days ago. 70°C idle temp (quite high, but unnoticable in everyday use),
90+°C with Reason running! The fucking thing was throttling itself so it didn't die. Hence the glitches even with just 1 or 2 instruments.
While at it, I replaced the old thermal paste between CPU and heat sink. Everything runs MUCH better now than before. I guess because of the old paste, I was used to some throttling now and then. System's now at 30°C idle temp and 45°C when Reason's blasting.
So excuse me while I'm off now, playing my songs at a buffer size of 128 samples just because I can.
TL;DR: Don't be stupid and change your thermal paste every few years!
Edit: Since I lost my bet, who wants my Aquios license? Nobody? Aww.