If you use iotop you’ll probably see btrfs-cleaner doing a lot of io deleting snapshots.
UrBackup doesn’t use the kind of operations that don’t scale well with a (moderate) amount of snapshots (e.g. btrfs send/balance etc.), so that isn’t the problem (except perhaps if you storage is more than say 90% full).
Your storage simply isn’t fast enough (w.r.t. random IOPS) to keep up with the amount of backups/data changes + deletion you want it to do.