Incremental backup stuck at "Pruning backup directory"

Hello, we've always had problems with backups. We do only 1 weekly backup (locally). Each backup took 3-4 days of increased server load, so we switched to incremental backups. It's the 3rd time an incremental backup is performed. The first one took as time as an uncompressed backup did. The second time the backup was relatively fast (~hours). This time the backup seems to be almost over but it is stuck at "Pruning backup directory" for 4 hours. Our load skyrockets (up to 50 for an 8 thread cpu) with a few small 5 minute windows of lower load. What's happening? Could I just stop and abort this process?