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variety of reasons.
a very simple test will be to take a level 0 archive to /dev/null. If the time is still slow then its probably due to either a lot of database activity or i/o transfer rate back from the storage where the db chunks reside. If fast then it's the transfer rate to the backup device that's probably the root cause.
Then again if your backing up to nas device then there is the network to consider.
you can use onstat -g stq to monitor the archive stream queue buffers. See the below link for more info.