qBittorrent official forums

There seems to be bugs with enabling torrent queues in general, and especially bad results if there's lots of torrents.

Sadly, you're better off as of qBT v4.1.5 with torrent queueing disabled and manually starting and stopping torrents...as messy as that can be with 2000+ torrents.

A lot of speed increases by ISPs tend to increase burst speeds but don't let you run those speeds for very long -- minutes to maybe an hour or 2 before congestion (contention with other users) probably occurs.

So how fast can you now upload and download at consistently? (This tends to be way lower than speed tests apps show!)

Hopefully you're running qBitTorrent v4.1.5 and update when the next version comes out. qBT v3.3.16 was reasonably stable as well, but lacks a few features (like disk coalescing) that improve speeds.

In BitTorrent settings...
If you're only doing private torrents, you can disable DHT, PEX, and LPD...but PEX is good to leave enabled if you sometimes use public torrents.
DHT uses far more connections and is also useful on public torrents that have dead trackers.
Local Peer Discovery is unlikely to find peers/seeds on the same ip range subnet as you (as though they're on the same LAN as you)...and those it does find are probably available through tracker updates, DHT, and PEX anyway.

Torrent Queueing has a few bugs that probably makes it better to stay disabled.

In Connection settings...
Disable uTP on busy torrents, because TCP speeds are much more reliable and faster. (qBT/libtorrent's uTP handling is buggy.) On torrents with few peers/seeds, you'll need uTP.
Disable UPnP/NAT-PMP -- if you're using a VPN or proxy, those are harmful because they forward the wrong network interface. (No need/point in forwarding qBT's listening port on your router if using a VPN or router.)
If not using a VPN or proxy, a manual port forward on your router/gateway is more reliable anyway.

In Advanced settings...
Disable "Resolve peer host names" -- these are often garbage and even sometimes faked to look like something else. It requires making additional connections to do as well.
Disable "Resolve peer countries (GeoIP)" -- these are sometimes wrong and not terribly helpful if you want more speed. It requires making additional connections to do as well.
Disable "Download tracker's favicon" -- this puts additional load on every tracker, which is especially bad if you are adding 20+ trackers to every torrent.

CORRECTION, NOT NECESSARY -> No need to disable "Send upload piece suggestions"
(I retested "Send upload piece suggestions" with qBitTorrent v4.1.5 and didn't experience the slowdowns I did in the past.)

Do NOT set cache size larger than about 1100 MB for qBT 32bit version! It can cause an out-of-memory crash. (32bit apps cannot use more than 2 GB normally.)
Set "Disk cache expiry interval" to 600 seconds (max allowed) to prevent the cache needlessly throwing away data -- it may help more than making the cache size larger than 500 MB.

Make sure "Always announce to all tiers" is checked and remove dead/irrelevant trackers from public torrents. (An irrelevant tracker is one arbitrarily added that is extremely unlikely to find other peers/seeds.)
Disable "Always announce to all trackers in a tier" -- that can actually get you banned from trackers, because you'd otherwise be announcing on their HTTP, HTTPS, and udp trackers all at the same time.

Private trackers check running totals from the client for current runs, but not past history.
So anything you download and upload while running over the last day goes on their record...they don't forget that when you reset your BT client.

Otherwise, everyone would risk being banned when they change BT clients or even reinstall their client from scratch.

The private trackers with ratio requirements are internally tracking your ratios separate from what qBitTorrent displays...after all, cheating BitTorrent clients could fake their ratios if they were only self-reported.

So the pain here is only that qBitTorrent isn't showing your real ratios to you anymore, which is annoying enough.

Possibly bufferbloat (when high DL speeds cause UL speeds to drop) and/or disk I/O overload (when copying from SSDs to to HDDs.)

If you only use private trackers definitely disable DHT.
PEX you can leave enabled because it won't do any harm if private tracker torrents are properly marked as private torrents.