Comment # 1 on bug 1143865 from Jeff Mahoney

So there are a few things going on here.

First, I think we need to report the error code that open_ctree failed with.  I
expect that systemd is killing the mount process and that's tripping a
mutex_lock_interruptible or something, so the mount fails entirely.

Second, what you're hitting is a known scalability issue in btrfs.  The block
groups are read in during mount.  Each block group is 1 GB in size, so you'll
have about 76k block groups.  The block group items are located in the extent
tree along with all of the extent records, so that means they are ultimately
distributed across the disk and are read in with what is effectively a random
I/O pattern.  Still, it shouldn't take 5 minutes to do it.

Is there any output in the log for that mount?