Performance issues and ping spikes in WDS environment

Hi, guys,

For a long time I have not considered this problem, but it drives me crazy.
I have three routers ap01,ap02 and ap03, the ISP router is connected to ap01 via ethernet.
The ap01 is WDS master (5GHz) and the other two are WDS clients.
Additionally there is a WLAN for the clients (2,4GHz) on each AP, this WLAN is the same on all three (same name, same password).

AP01.wlan (bridge 2,4Ghz and 5Ghz)
AP02.wlan (bridge 2,4Ghz and 5Ghz)
AP03.wlan (bridge 2,4Ghz and 5Ghz)
ISP router

Actually everything works, everybody can communicate with everybody, everything is good.
But the performance sucks, if there is nothing going on in the network and I am connected to my PC on ap03 and ping on the lan ip of ap01 (PC= -> ap01.eth=, then I have about 1-4ms latency, which is ok.
But if I run a Youtube video on the PC I have a latency of 120ms avg on the same ap01 at 10 pings.

I know that I have not given you much information yet, I wanted to explain the topology first.
On all routers OpenWrt 19.07.2 is running, I have no QoS activated, all interfaces belong to the same FW group.

Please help me.

First question... Why is every ap on it's own subnet?

What is more important, speed or latency?

What is you internet speed?
Can you measure you avg throughput over the wireless links?

Behind each router there are other devices at the switch, so I didn't want a flat network. On ap01 is the workroom, so NAS, printer and desktop. On ap02 is the living room and on ap03 the guest room. In the course of time I wanted to install some ACLs there.

I think a good mixture of latency and speed is ideal, I'm not a hardcore gamer or downloader.

Internet is 50/10 Mbit

I can measure bandwidth, one PC to the WLAN of ap01 and one to the WLAN of ap03 and a direct file transfer.
But that means that the traffic from the pc via 2,4 ghz to ap01 arrives on wlan0, has to go internally to wlan1, then the WDS (5GHz) goes to ap03 and there again from wlan1 to wlan0 and then over the 2,4 GHz to the PC.

The testing is done, i have used an FTP client/server combination to test and one file with a size of aprox 1GB.

both PCs in WLAN of ap01 - transferspeed of 5,0MiB/s avg
one PC in WLAN of ap01 and the other in WLAN of ap03 - transferspeed of 2,6MiB/s avg

Does anyone have an idea, where to start?