Massive BTRFS performance degradation

[KC <impactoria@googlemail.com>]

I am experiencing massive performance degradation on my BTRFS root 
partition on SSD. Except for regular daily updates, nothing changed in 
the system. The mount point remained the same:

/  btrfs rw,noatime,compress=lzo,ssd,space_cache,autodefrag 0 0

but the performance dropped to less than 8% of norm.


Before:

# dd if=tempfile of=/dev/null bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 1.57307 s, 683 MB/s

Now:

# dd if=tempfile of=/dev/null bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 26.4373 s, 40.6 MB/s


I created a new btrfs partition on the SSD with the same mount options 
and it is not being affected:

# dd if=/mnt/er/tempfile of=/dev/null bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 1.57634 s, 681 MB/s


I also did
btrfs filesystem balance start /
wit no effect.

I tried changing mount options - still no effect.

I'd appreciate some suggestions.