[Shotsgrin]

RG Premium user and I am also having this exact issue. Right at 80%, regardless of file size, slows to 8-10 kb/s.
Reproduced on both windows 10 / mac0s 11.2.3. Cox ISP. Also happening with VPN (PIA).

Update: RapidGator.experimentalenforcessl might have worked on macOs. Running more tries.

Update 2: Three successful tries on macOs after enabling force ssl option. 1.3GB, 1.3GB and 2.06GB, no vpn. Of note, seems to still slow a bit around 80% but jumps right back to normal speed. Windows 10, three unsuccessful tries after enabling force ssl option. Slowed at 60% twice and once at 40%. No vpn. Stop and restart solved, same as before.

Update 3: Just tried on different mac w/ 11.2.2 and had a slow down at 60% after enabling force ssl. Back to the drawing board.

So to summarize: Using the same ISP, Router/Modem (Arris), VPN/Non-VPN, RG PRemium account, I have seemingly solved the problem on one Mac (mac mini, 11.2.3), and NOT solved on Windows 10 (19041.867) or Mac (MacBook Pro, 11.2.2).

Nix that - just had a slowdown at 80% on the Mac I thought the issue was resolved on. OK... that is all the info, experimenting I've got. Hope at least some of this info helps.... this is super-annoying.