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What should I set the disk cache size to? I am on Win10 x64 with 8GB of memory.
The HD where I am putting my torrents is relatively old and slow. Should OS cache be on or off?
Right now it's at 64MB disk cache with OS cache on.

For me it's about "optimizing" my upspeed for seeding, don't care too much about download speeds. (500/500 fiber). Currently seeding about 30 torrents.

I'd also like to know why the nr of seeds & peers in brackets is usually MUCH higher than the actual numbers. I understand these are estimates and show the theoretical (?) nr.in the swarm, so it is normal that these nrs. are usually higher?

It is normal for the seeds+peers numbers in brackets to be greater, especially on public torrents.
Peers download quickly, become seeds, and then quit running the torrent but still remain counted as "seen" on the torrent swarm.

On really large torrents, peers and seeds stick around a little longer.
Some may be changing ip addresses every few hours to days, so get "seen" multiple times.
Quite a few may have both IPv4 and IPv6 addresses and at least get counted twice.

For the disk cache, there's only so much you can do with a very slow HDD and a very fast internet connection.
Maybe increase the cache duration from the default of 60 seconds to 600 seconds.
Do a Scandisk and Defrag the drive -- this should make sure the existing torrents can be read quicker and not delay downloading torrents as much.

If you can, download to a different drive then have qBitTorrent auto-move to the slow drive when finished. That can almost eliminate file fragmentation IF you previously defragged the slow drive.
I use a ramdrive (2-7 GB size) for this purpose, although only the small torrents can take advantage of this -- the bigger torrents I can download to C: then move to D: