you have to find if your CPU or the I/O is the issue. you can monitor your server with top and watch the columns '0.0%us' and '0.0%wa'.
then if possible, you can try lowering the cpu or io priority of the backup process and see if that will help you.
in general tar, especially when passing the compression to gzip is known to be causing high cpu spikes.
re-nicing the proccess on cpu and io may help sometimes.
check 'ionice' and 'nice'