Something in the old backup probably is corrupt.. it takes a lot of time.. as long as the original backup or longer to verify the backup and then has to start over if it finds too much corruption and has to do the backup all over again.
Wireless is a poor media for this. The TC should be located much closer to the computer so you can use 5ghz and get high speed. In the end turning off TM to save time actually works against you.. when it does a backup the actual verify takes ages.
If you cannot get the TC closer think about using a different method.. eg a USB disk plugged into the laptop. It is much faster and will cause no slow down in the network.