Slave (SQL thread) replicates very slowly
(Too many questions for 'comments'.  Anyway something here might help locate the naughty.)

Backup occurring?

sync_binlog - Probably 0 is OK for Slave.

innodb_flush_log_at_trx_commit - may as well be 2 on a Slave.

Differences in OS config, especially relating to I/O?  Swappiness?

Are clients hitting both Slaves?  In the same proportion?

How much RAM?

query_cache_size        = 512M -- This is bad for any system.  Limit it to something like 50M.

tmp_table_size=1G -- This is dangerous.  Some complex queries could chew up RAM and cause swapping, which is really bad for performance, especially I/O.  Keep that down to, say, 1% of RAM.

You have the slowlog turned on -- Look for clues there.  If you don't find anything, then turn ON log_slow_slave_statements also.

Is Seconds_behind_master increasing or oscillating?