It surprises me it’s that big of a difference, are you sure discard was the only thing you changed between those ppd numbers?
What is the iowait % with and without discard? That is imo the best indication if you’re io bound.
If you don’t send TRIM commands, which is what discard does, the SSD doesn’t know only 60-70% is in use. That’s why you want to enable discard.
The alternative is to regularly run fstrim on your temp drives. I don’t know how often is needed. But once a week, which the the default in Ubuntu iirc, is nowhere near enough. We fill the whole drive on an hourly basis, fstrim should be more often that that.