MySQL binlog rsync from tmpfs to HDD causes write slowdown on ramdisk

I have a mysql database server and am testing writing my binlogs to ramdisk to speed some things up. A curious thing happens.. fairly often, I get major job slowdown at the same time the rsync is happening. Updates that take almost zero seconds (when the binlog is writing to an ext4 fs that has been optimized for write), take upwards of 2 seconds (which is a big deal with the number of updates I am doing). This surprised me, as rsync is not supposed to be doing any locking anywhere, and since the tmpfs has a throughput of 2GB/s (and the rsync is much less than a gigabyte each pass)

Can anyone shed light on this one for me? How is the rsync from tmpfs causing write slowdowns on my binlogs, and thus update slowdowns?