I connected RouterBOARD 260GS between 1GBit LAN Lenovo computer and RB SXT 5 ac .
All devices are declared 1 Gbit LAN, but all reports speed only 100 Mbit !!!
Lenovo to RB 260GS is 40 cm long cable and RB 260GS to RB SXT 5 ac is 2 m long cable. What is wrong ???
I need Gbit connectivity and therefore bought expensive RB 260GS !!!

So, to start, it looks like your SXT isn't advertising at 1000M. Check the "Ethernet" tab for your SXT interface and ensure that 1000M full and Auto Negotiation is checked. Can't say what's going on at the Lenovo...perhaps you've got an outdated driver, or maybe a bad cable. 100M ethernet only uses 2 of the 4 pairs, so you could easily have a bad cable and never know it if you're only connecting at 100 (assuming all is good with the NIC being 1000M and having the correct drivers installed).

SXT had not set Flow control at "Auto" by default, there was "off", changing it to "Auto" opened port at 1Gbit. Problem with Lenovo persists on all settings the integrated LAN card. No tragedy, it is notebook in docking station, it will be not at place where network will be placed... maybe Lenovo problem. I was afraid, that Mikrotik is not compatible with self... Thanks for help!