Hi all.

So, as the title says - I'm experiencing slow LAN to Internet performance between a HAP AC2 router and a RB260GS switch.

My current setup is two different computers (1 Ubuntu Server and 1 windows Desktop PC) connected to the RB260GS switch, which is connected to the HAP AC2 router, which in turn is connected to my ISPs modem.
After a reboot of either the RB260GS or the HAP AC2 I can do a speedtest from either of the two computers connected to the RB260GS to https://www.speedtest.net/ and get my ISPs advertised speed (~300Mbps download/~100Mbps upload). But, after a while I only get (~100Mbps download/~100Mbps upload) - and I can't figure out why.

Troubleshooting so far:
I've checked that the HAP AC2 is not under any heavy load - neither CPU, RAM or otherwise. I've had a laptop connected directly to the HAP AC2 and tried a speedtest while the computers behind the RB260GS where experiencing the slow speed (~100Mbps download/~100Mbps upload), and the laptop directly connected to the HAP AC2 got the "correct speed" (~300Mbps download/~100Mbps upload). So, my thinking is that this is somehow an issue between the HAP AC2 and the RB260GS. After a reboot the speed is back to the "correct speed" - but after some time (between an hour and several hours) the speed just drops to ~100Mbps download/~100Mbps upload.

Any help, ideas or explanations would be appreciated.

First of all check what is the negotiated speed on the ethernet ports - it may renegotiate to FE (100 Mbit/s) from GE (1000 Mbit/s) aftre some time of running on GE if there is something wrong with the cable or connectors.

Since I'm at work, I can't check what the negotiated speed between switch and router is right now, I will check when I'm home.

A quick speedtest-cli from the Ubuntu Server showed that it's currently at the slow speed and when I check with ethtool on the relevant ethernet interface the output is as follows: I believe I checked on the RB260GS what the negoitated speed was yesterday night when the problem was there, and I'm fairly certain it said 1000MB or 1GB on all ports / the ports in use (two computers and the port to the Hap Ac 2).

The situation on the cable between the PC and one of the Mikrotik gives no information about the situation on the cable between the two Mikrotiks.

If you find the negotiated speed to be gigabit and the test shows less than 100 Mbit/s, check CPU load on the hAP ac² while testing (/tool profile), but as you say that it is enough to reboot any of the two devices to fix it, I bet on the Ethernet speed renegotiation.

First of all check what is the negotiated speed on the ethernet ports ...

Not to get lost in all different places where speed is mentioned and is not relevant to the problem ... the correct command to check current negotiated speed is