RB/493G Wire performance issues

Hi all,

Ive recently got an RB 493 G with 2 R52 Hn running RouterOS v5.21.

Regarding the wire throughput I think its not optimal yet.

When transferring files between a desktop pc and the NAS I get speeds
around ~50-60 Mbyte/sec. The transfer starts at about 90-95 Mbyte/sec
(what I would expect from this setup) but after half a second it drops to 50-60.

With another soho router/switch the speed was between 70-80 Mbyte/sec.

Ive tried to connect the hosts to different ports (switch-cpu1 and switch-cpu2)
Also the auto negotition works.
During the file transfers the Mikrotik CPU load is between 1-2%.

Configured features are : DHCP, Masquerading and a bridged PPPOE WAN Interface.

Is there any configuration I could try to improve the performance ?

Thanks in advance!