Ah, slow. It exists in a sort of grayish limbo between status and stat bust, and varies widely in effectiveness. I've been thinking recently: How advantageous is it to have a very powerful speed bust?
Bear in mind I'm only talking about the kind that interferes with your ability to get turns. Things that mess with speed/agility stats that only govern shit like evade and move don't count.
So, I'm thinking: What game's bosses block it? Which explicitly don't? Which, if any, are hard to call?
Ones I remember off the top of my head:
BoF1: Idle fucks them over, they're vulnerable.
SMT games: No real slow in these, but it should almost certainly be assumed to hit them because every other stat down works on all of them.
DQ8: Depends on the boss. Empyrea's immune, others are varying degrees of resistant, IIRC.
FFT: Also varies, I believe?
FF6/7/8: Immune
Grandias: Affects all of them, though some of G3's recover from it quicker.
Lunar 1: Doesn't matter
MK: Immune, I believe
Saga: Vulnerable
SMRPG: No slow status, but you might extend Fear immunity/vulnerability to affect stat downs in general, so it depends on boss. But none of them are even ranked, so w/e.
SoA: Immune
VPs: Vulnerable by SMT clause
WA1, WA4: Vunerable