If this behavior is intermittent as you indicated, then most likely something else is using resources at the time when the slow backup job is running.
This could be another backup job; this could be something on the BE server itself like AV scan; something on the target destination, something on the source server itself or some other application does something which uses the network.
To really understand what it could be you need to investigate this in more details.
Does it affect all jobs when it happens or just one job?
It would make sense to monitor this closely or go back in time and write down on which dates this issue is seen.
Maybe its always on a Wednesday? or the same two days? Often there is a pattern of some sort.
My apologies for the late response.
All backups from april 8th until june 7th have been slow.
The backup of yesterday is again a fast one.
It's still the same configuration, the same amount of data, ...
The backup job runs on a server with a tape-streamer built-in, so no network interference.
No other backup jobs at the same time.
No pattern what so ever.
Is this slowness only seen for this one job?
Other jobs do not show this slowness and always have the same speed?
I would expand the last two job logs (quick and slow) and compare each resource in regards performance.
If all resources are showing the same reduction of performance then its something on the server level;
If only one resource is affected, then concentrate what is happening to that resource and maybe share that info with us (what that resource is).