Hi.
We have several switches: CRS354-48G-4S+2Q+ ( 7.15.1 ).
Several ports have a problem: download speed is approximately 10Mb/s (speed of connection is 1Gb/s). When I change port (for example from 36 to 34) - its ok. Incoming and outgoing traffiс is all right.
It is not a problem of patchcord and provider or QoS. Theare are several vlans on the switch. Very simple config.
So how can I rid of this problem?

Are you using RouterOS or SwOS?
Can you share the config of the switch?

Could there be any queue active on the port (or VLAN)?

Using ROS.
The problem in on the port 34. Temporary I moved connection from 34 to 36.
Config is minimal. I didn't config any queues...

Config is:
/interface bridge
add name=bridge pvid=10 vlan-filtering=yes
/interface vlan
add interface=bridge name=vlan10-management vlan-id=10
/interface wireless security-profiles
set [ find default=yes ] supplicant-identity=MikroTik
/port
set 0 name=serial0
/interface bridge port
add bridge=bridge interface=sfp-sfpplus1 pvid=10
add bridge=bridge interface=sfp-sfpplus2 pvid=10
add bridge=bridge interface=ether1 pvid=10
add bridge=bridge interface=ether3 pvid=10
add bridge=bridge interface=ether5 pvid=10
add bridge=bridge interface=ether7 pvid=10
add bridge=bridge interface=ether9 pvid=10
add bridge=bridge interface=ether11 pvid=10
add bridge=bridge interface=ether13 pvid=10
add bridge=bridge interface=ether15 pvid=10
add bridge=bridge interface=ether17 pvid=10
add bridge=bridge interface=ether19 pvid=10
add bridge=bridge interface=ether21 pvid=10
add bridge=bridge interface=ether23 pvid=10
add bridge=bridge interface=ether25 pvid=10
add bridge=bridge interface=ether27 pvid=10
add bridge=bridge interface=ether29 pvid=10
add bridge=bridge interface=ether31 pvid=10
add bridge=bridge interface=ether33 pvid=10
add bridge=bridge interface=ether35 pvid=10
add bridge=bridge interface=ether37 pvid=10
add bridge=bridge interface=ether39 pvid=10
add bridge=bridge interface=ether41 pvid=10
add bridge=bridge interface=ether43 pvid=10
add bridge=bridge interface=ether45 pvid=10
add bridge=bridge interface=ether47 pvid=10
add bridge=bridge interface=ether2 pvid=20
add bridge=bridge interface=ether4 pvid=20
add bridge=bridge interface=ether6 pvid=20
add bridge=bridge interface=ether8 pvid=20
add bridge=bridge interface=ether10 pvid=20
add bridge=bridge interface=ether12 pvid=20
add bridge=bridge interface=ether14 pvid=20
add bridge=bridge interface=ether16 pvid=20
add bridge=bridge interface=ether18 pvid=20
add bridge=bridge interface=ether20 pvid=20
add bridge=bridge interface=ether22 pvid=20
add bridge=bridge interface=ether24 pvid=20
add bridge=bridge interface=ether26 pvid=20
add bridge=bridge interface=ether28 pvid=20
add bridge=bridge interface=ether30 pvid=20
add bridge=bridge interface=ether32 pvid=20
add bridge=bridge interface=ether34 pvid=20
add bridge=bridge comment="only for test. patch from 34 port" interface=ether36 pvid=20
add bridge=bridge interface=ether38 pvid=20
add bridge=bridge interface=ether40 pvid=20
add bridge=bridge interface=ether42 pvid=20
add bridge=bridge interface=ether44 pvid=20
add bridge=bridge interface=ether46 pvid=20
add bridge=bridge interface=ether48 pvid=20
add bridge=bridge interface=sfp-sfpplus4 pvid=20
/ip firewall connection tracking
set udp-timeout=10s
/ipv6 settings
set disable-ipv6=yes
/interface bridge vlan
add bridge=bridge tagged=bridge,sfp-sfpplus1,sfp-sfpplus2 untagged=\
ether1,ether3,ether5,ether7,ether9,ether11,ether13,ether15,ether17,ether19,ether21,ether23,ether25,ether27,ether29,ether31,ether33,ether35,ether37,ether39,ether41,ether43,ether45,ether47 \
vlan-ids=10
add bridge=bridge tagged=bridge,sfp-sfpplus1,sfp-sfpplus2 untagged=\
ether2,ether4,ether6,ether8,ether10,ether12,ether14,ether16,ether18,ether20,ether22,ether24,ether26,ether28,ether30,ether32,ether34,ether36,ether38,ether40,ether42,ether44,ether46,ether48 \
vlan-ids=20
add bridge=bridge tagged=bridge,sfp-sfpplus1,sfp-sfpplus2 vlan-ids=21
/ip address
add address=192.168.203.9/24 interface=vlan10-management network=192.168.203.0
/ip dns
set servers=8.8.8.8
/ip hotspot profile
set [ find default=yes ] html-directory=hotspot
/ip route
add disabled=no dst-address=0.0.0.0/0 gateway=192.168.203.1 routing-table=main suppress-hw-offload=no
/system note
set show-at-login=no
/system routerboard settings
set boot-os=router-os enter-setup-on=delete-key

I have 2 vlan's. One for management, (vlan10) and for internet connection (vlan20).
I didn't check all ports. Only 34 and 36. 34 is bad, 36 is good.

Most of the ports working normaly.

The problem eliminated by switch reboot.

Have you tried 7.15.2?