Are you using SL or Lion??
I think there are severe driver issues with wireless in Lion.. especially upgrade not straight install.
Try using ethernet.. but we are seeing a number of people with slow issues.. if it is still slow on ethernet and you have applecare, get apple involved as it is their faulty buggy firmware that is the cause.
In the meantime, bridge the TC and keep using the FIOS wireless.. there is nothing wrong with that approach and you can still access the TC via the provided router.
The Time Capsule is a slower device than your router to which it's connected so the router must throttle down to th same speed as the slowest device on the network.
Rated speed of the Latest TC and AEBS is 430mbps WAN TO LAN.. way in excess of the fastest possible speed on cable today..
The failure is a network issue although I do not know the cause.
Bridge means to remove all routing from the TC.
Open Manual setup in airport utility (on lion v5.6 utility only).
Go to internet tab.
Go to Connection sharing.
Select off, bridge mode.. update the TC.. it is now bridge.
If still slow there are a number if issues.. and I doubt we have found the ultimate cause.. bad firmware in 7.6 could be one of them .. go back to 7.5.2
ie in airport utility go to the firmware update but hold down the option key and you can select older firmware.. they work better than new ones.
You also might need to reset the TC to factory to get rid of the poor settings.