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?

It's not a fancy setup, Technicolor router 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?