qBittorrent official forums

I've noticed that after limiting upload speed, download speed dramatically falls.

For example, I'm downloading at 8 MiB/s and uploading at 2 MiB/s. Then I choose to limit my upload to 1 KiB/s and the download speed goes down to 560 KiB/s (all speeds are approximate). Then I set upload limit again to "infinity" and download speed goes up again.
I am wondering why is this the case.

I'm using qBittorrent 4.1.5 64-bit on Windows 10, 1809, 17763.437.
All settings in qBittorrent are default ones.

"Is download speed dependent on upload speed?"

Yes, in multiple ways and for multiple reasons.
TCP/IP overheads and BitTorrent overheads just for starters.
Just requesting the next pieces of a torrent to download from a seed costs a little upload -- so setting upload limit to 1 KiB/s cripples even that!
Seeds and Peers will snub (and refuse to upload to your peer) if you set qBT's speed limits too low.

On torrents with few seeds but many peers, your download greatly depends on your upload to other peers...since BitTorrent has a Tit-For-Tat (something-for-something) reward system built into it at the torrent level.
Peers tend to upload mostly to the OTHER peers that are giving them the most download.

Thanks for the info!

It's always cool to learn something new.