Your host could be auto negotiating the speed with up stream switches.please check the switch port configuration like speed, duplex settings end to end
I checked - everything is fine here. But I found the reason: "Beacon Probing" was specified in the settings of the "Teaming an Failover -> Networ failure detection" port-group. I installed "Link status only" and the copy speed became high.
However, when copying a file, this speed is high only at the beginning of copying, in the process it steadily decreases (from 300 megabytes / sec to 30).