I have finally managed to figure out what was causing the problem. While I tweaked several adapter settings as well, things only started to work properly after I enabled Windows AutoTuning, which had been disabled before.
According to my ISP plan, my upload speed should be 100Mbps or 12.5MB/s. In a speed test, I can usually do a little better, but it varies throughout the day. With AutoTuning disabled, I could upload an individual file through rclone at 2.5MB/s max. With AutoTuning enabled, I can now reach about 11.5MB/s for that same file. This isn't 12.5MB/s, but it's close enough. Besides, there are other network adapter and Windows settings that could probably be optimised better for a fiber connection. I read several guides about this before (for both Windows 7 and Windows 10), but unfortunately virtually all of them give conflicting information on what the best settings are... Any suggestions in that area are very welcome.
Anyway, to check the status of Auto-Tuning feature on your system, in an elevated command prompt windows, type the following and hit Enter:
netsh interface tcp show global
To enable Windows AutoTuning, run the following command:
netsh int tcp set global autotuninglevel=normal
Maybe this is of some use to someone in the future.
Finally, I would like to thank again all the people who tried to help me with this issue in this and another topic, where it was also discussed.
UPDATE: Apparently, it is necessary to disable Internet Protocol Version 6 (TCP/IPv6) in addition to the above, because it won't work otherwise. This can be found on Windows 7 (which is the OS that I am currently using) in Control Panel -> Network and Internet -> Network Connections, then select the relevant network interface controller/network adapter and go into Properties. Goddamn Windows and goddamn computer stuff...