Why does the speed drop when the LAN-LAN router is turned on?

Question

Computer networks

Hello!
There is a network from 2 routers. Optics comes first, wifi is distributed from it and a wire is connected to the LAN port that goes to another room to the TP-Link TL-WR841N router, which, in turn, distributes wifi in this room. When you connect a wire to TP-Link's WAN port, the speed of the devices connected to it is approximately the same as on the main router where the optics come from, but if you connect the wire to TP-Link's LAN port (disabling DHCP), i.e. when using it as a switch, the speed in the subnet with TP-Link drops dramatically (instead of 40 Mbps, it turns out, at best, 1 Mbps).
I want to connect in the second way, through a switch, so that there is one network and you can use the file server located on the first subnet.
I will be grateful for help.