For a test, does a computer hardwired directly to the router / gateway device with an ethernet cable have the same problem ?
I have the same issue. I'm on my 2nd replacement modem. I have a bunch of equipment, including 2 wireless routers, wired in to the ethernet port on the modem and it's all intermittently slow now. I've just added a wireless access point configured in client mode to avoid using the ethernet ports on the modem but it doesn't seem to have helped much.
Some tips for improving WiFi performance;
There are other options. Google for WiFi repeaters / range extenders, WiFi mesh systems / pods, powerline ethernet adapters, MoCA adapters. Good luck !