Hi \-V-/,
Once TM has done its initial backup and Spotlight has indexed all of the disks, then TM should not be much of a drain on the computer's resources. This is because the index allows TM to backup just what has been changed since its last backup without TM needing to "search the entire disk" for what to backup. TM only refers to the index to know what files need backing up, and thus this requires very little computer resources.
In other words, once TM and Spotlight are "up and running", so to speak, there will be a minimal impact on the computer's resources and speed when doing TM backups. Of course, the TM backups, if done wirelessly, do take up some wireless bandwidth, and this you might notice, but since most hourly TM backups are minimal in size (they are differential, not full, backups) the amount of data transferred wirelessly is quite small and so shouldn't have much of an impact on your wireless either.
Just thought you would like to know...
Regards,
Switon