Did you perform a complete, full power cycle of the entire network?
If not, power off every device on the network in any order that you want
Make sure that the modem is fully powered off by pulling the power cord from the back of the modem or unplugging the power supply from the wall AC jack
Wait 10-15 minutes, longer would be better
Start the modem first and let it run 3-4 minutes by itself
Start the Time Capsule next the same way
Continue starting devices one at a time about a minute apart until the network is back up
Check the network
If you want want stay with the new firmware, all you can do is perform a Factory Default Reset on the Time Capsule to start over again from scratch. I'm not really recommending that you do this, because there are no guarantees that it will help, but it will sometimes help.
If you "upgraded" to Firmware 7.6.1, that is not one of Apple's better efforts. You could try rolling back to the previous version.
If you roll back to the previous version of firmware, and the speed reduction continues, unfortunately the network gremlins have arrived at your home.