OpenWrt (with default settings) causes (?) torrent speed fluctuation. What could be the problem?

Hi!

I've just finished my first ever installation of openwrt to my new linksys mr8300 router. Everything seems fine except for one thing: my linux laptop which I use for torrenting (connected with cable to the router) has insane speed fluctuations. For example it's seeding with 2.4MB/s, then goes down to 200K/s, then goes to 600K/s then goes to 0, then back and then to zero etc... So it 99% not because of a strange anomaly with the peers.
However download seems to be stable.

Any thoughts on what could cause this? I didn't make any changes in openwrt other than setting up guest wifi which is not related because this computer is on cable.

Is there some strange default limit to connection numbers maybe?

I'd appreciate any help! Thanks!

edit:


(so I guess it's not because of connection numbers, but this has to be global, maybe it restricts individual clients?)

edit 2:
I see a similar issue here: Constant high and low speed peaks with torrents
I wanted to attach a similar graph as well, but I don't see how I can filter for only one IP unfortunately and I have multiple devices on cable which can give noise. Still trying to find a way to make it somehow.