Is that a wireless router?

Yes, but pretty much all the wireless connecting goes on with an access point in another part of the building.

 

Not the full network but this will give you an idea:

 

Access Point-------Switch-------Router
|
|
Other Office switch

I'm with you - it could certainly be your router, but I think you need to spend a little time narrowing things down between other devices.

 

Though as Ant says, could easily simply be your internet speed. That said, to be fair, day to day internet use doesn't use quite as much as you'd think. I know of places with 500+ machines using 10mb - 20mb with no drama. But then it does depend a lot on the quality of your connection, contention rates etc etc.

If your router and switches provide SNMP access, get hold of the free version of something like PRTG and get some bandwidth monitoring going.