RDP over L2TP IPSec really slow

My current setup:

(1st floor:) Modem --> pfSense box --> Archer C7 (OpenWrt 18.06.5) --> (2nd floor:) TP-Link TL-SG108E --> (3rd floor:) Archer C7 (OpenWrt 18.06.5)

I'm running 5 VLAN's. Both the Archers are setup with 4 SSID's on both bands (2.4 & 5GHz) so 8 wifi networks in total per device.

For work I connect to a L2TP over IPSec VPN with Split Tunneling (local traffic over VPN only), from there I can connect to the RDP servers at work.
When I'm on the 3rd floor and connected to my wifi_admin network I have a strange problem.
After connecting to a RDP server (over L2TP) the image composition is slow as f*ck. It's so laggy it's unworkable. When I change to another SSID on that same Archer the problem is gone.
When I go downstairs and connect to the wifi_admin netwerk the problem is gone as well.
I'm connected with a link speed of 867Mbit everytime.

So the problem seems to be only when I'm connected to SSID "wifi_admin" and only when I'm on the 3rd floor, connecting to the Archer there. Also it's only happening to RDP servers I access behind the L2TP VPN. Other RDP servers which I connect to over the internet (no VPN) don't have this problem.
I have checked all settings and they all are the same for both Archers, except for the channels. I use different channels on the 1st floor than on the 3rd floor.

Does anyone have a clue what's happening here?

Could you have a conflict between the IPs on your network and the IPs at work?

No, all are different subnets.
Also when connected to the same SSID, but downstairs instead of upstairs the problem is gone.
Of course I'm getting the same IP up or downstairs as it the same client machine (Macbook Pro) I'm using.

Are the routers connected by cable or wifi?

(1st floor:) Modem --> pfSense box --> Archer C7 (OpenWrt 18.06.5) --> (2nd floor:) TP-Link TL-SG108E --> (3rd floor:) Archer C7 (OpenWrt 18.06.5)

All cat6 cables between them.

I'm out of ideas here...

Perhaps a MTU issue?

Could you double-check that this happens only on RDP connections from that SSID on that router?

I'd be digging out Wireshark to see if there is a pattern revealed. I'd be looking for changes in packet loss or the like.

Guys, I'm really sorry.
But the problem was due to an issue with my ISP.
It really seems to be a problem when I was connected to that one SSID but in fact it was not.
Sorry for the troubles. It's fixed now.
Thanks for all the help!

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