Logically, the first backup we make using Time Machine to a disk connected to an Airport Extreme or another mac must be slow, so slow that it can take between 5 and 8 hours the first time you make the copy.
The trick is to start copying by letting the preparation process pass on the remote Time Machine disk and stopping it and then unmounting that disk and re-mounting it on a local USB on our Mac.
If we had the disk on another mac we will have to tell Time Machine to change disk, it will find the job started and will continue with the copy and it will be ready in 1 or 2 hours. Then we return the disk to its place and mount it. If necessary we will tell Time Machine to use the remote disk again.