I am referring to the issue noted here: https://forum.mikrotik.com/t/rb5009-slow-speed-2-5g-bug-report-as-requested/155362
I am experiencing the reverse of what the above poster experienced, but the issue seems to apply in either direction.
All the link speeds you asked about are appropriate. My internet connection is 2 gigabit, so I have a 2.5 connection from the modem to the switch. Then 10gb between the switch and the router. As for the PC, I have tried both 1 gigabit and 10 gigabit connections. The 10 gigabit connection gives me full internet speed, but the 1 gigabit connection only gives me 500-600 mb.
Based on what I've been reading, this appears to be something to do with the buffer in the router when moving between different port speeds.