R D S wrote

I've 80mb fibre at my workplace, and I get almost full speed.

But it gets slower and slower over the course of several days to the point
of unusability, then I reboot the router and it's flying again.

What would cause this? Stuff beyond my control, or is it possible it's my
gear?

Has to be your gear given that a router reboot fixes it.

It's not a fancy setup, Technicolor router

Dont see that here with the Technicolor TG789vac v2 HP

with a couple of PCs plugged in and a downwind switch with a PC and a
printer plugged into that.
Laptop and mobile connecting wirelessly.

What could cause a network to clag up?

Gotta be the router.