Quite strangely, the day after my first message, my upload speed came back to normal (~ 500 Mb/s) and remained such till today.
I've waited 3 days before experimenting to freeleech anything; I chose to freeleech a rather small torrent today.
At the very moment I did that, my upload speed fell again at <1Mb/s!!! (I've checked on www.speedtest.net and some other similar sites.)
I'm left with two hypotheses:
— my ISP is delivering an instable upload trafic when I freeleech something (3rd time in a row…);
— there's an obscure unexpected interaction between gaytor.rent & my fiber box (the speed of transfers between the computer and the box, using FTP, doesn't seem to be affected)… 🤯