I am using a CCR2004 as my main router. My ISP has provided me with a /31 point-to-point WAN transport link, as well as a separate routed /29 subnet. The ISP connection is delivered via a SFP+ handoff and it goes straight to the CCR2004's sfp-sfpplus1. I have completed the usual setup (with the /31 workaround for Mikrotik), manually added a 0.0.0.0/0 route pointing to the other end of the /31, configured NAT masquerade for sfp-sfpplus1, and everything up till that part is working great, with all NAT clients getting the full 10G speed.

I've moved on to configure the /29 network on the sfp-sfpplus12 interface, which is then bridged to a CSS326 switch using a 10G jump cable. The /29 subnet is excluded from NAT translations. I connected a computer to the switch port and manually configured the static IP and gateway settings. Internet works and there are connections both ways, the computer can be pinged externally and visiting ifconfig.co shows the IP I've configured for the server, so that's good too.

The problem, however, is with the throughput -- with the same Speedtest server used previously, I'm only getting 200-300Mbps for download and upload, instead of the expected 900+Mbps (the node is connected to a 1G port of the switch, and the switch has a 10G uplink to the router), which is very strange. On the WAN port, the CCR2004 should be delivering more than 9000M throughput to the switch, as tested previously with a direct connection from a PC to one of the SFP+ ports on the CCR2004, with a NAT connection. The CCR2004's CPU usage during the Speedtest was around 0-2%, and under Winbox's Interfaces view, the sfp-sfpplus12 connection status is reported to be at 10G full-duplex, without any collisions or errors. I have repeated the tests many times, and the results are similar.

Here are what I have tried but to no avail:

• Changing to a different port on the CCR2004

• Using a RB4011, configured as a switch, in place of the CCS326

• Using a different set of SFP+ modules and cable

• Using a different LAN cable connecting to the server

• Changing the /29 address of the router

• Adding fasttrack connection rules for traffic forwarding to and fro the /29 subnet

• Disabling all firewall rules

• Disabling all srcnat rules

• Disabling all interfaces except sfp+1 and sfp+12

• Using a different computer

Both the CCR2004 and RB4011 are on firmware 6.47.2, whereas the CSS326 is on SwOS 2.12
What else could I have missed here? I've been spending the past two days trying to troubleshoot... Any help would be greatly appreciated!

[Update 4 Sep 2020] I've updated the CCR2004 firmware to 6.47.3, but that didn't change the routed /29 throughput.

[Update 8 Sep 2020]

As it turns out, it is a case of the Mikrotik CSS326 or CCR2004 being picky with the GbE adapter of choice.
Swapping out the network adapter restores full link speed immediately. The same "slow" adapter could obtain full speeds with other network equipment, which is why I wasn't suspicious about it at first.

Would you exactly name the adapters, please?
The working and the non-working.