Disk I/O increased when changing RBR to MIXED
When a complex UPDATE is run on the master, and it results in only a few rows being updated, then one of these happens:


SBR:  Re-execute the UPDATE on the Slave.  This stalls replication, waiting for the complex query to finish.
RBR:  Only the changed rows are sent to the Slave.  These can be applied much more rapidly.


A simple UPDATE that modifies a million rows:


SBR:  Again, slow on the slave -- because of all the I/O do update the million rows.
RBR:  This is also slow, but for a different reason:  A million records are sent through the replication stream.  This involves I/O at both ends, plus network traffic.


DELETE, INSERT...SELECT, and a few other things have similar characteristics.

I can't be more specific without knowing what, exactly, you were replicating.  I hope my examples give you some clues of what could be happening.