Does it make any sense to put cards with many "Again"s into a separate deck? If I don’t, how do the slow cards influence the progress of the fast ones? I recently started using FSRS.
No, I don’t think so. FSRS knows how to treat those “slow” cards differently given their own review history (that’s sort of its superpower!), so there’s no need to segregate them. They won’t hurt “fast” cards at all.
If there is some systematic difference to those cards that would cause the FSRS params to be different there could be a reason to as putting them in a deck with a separate preset would allow you to optimize separate FSRS params for those cards. This is provided you have sufficient review history for them (ie. >1000 reviews).
Otherwise I don’t think it matters.