Very high btr_search_latch after upgrading from 5.6.40 to 5.6.44

That statistic is related to the innodb adaptive hash index https://dev.mysql.com/doc/refman/5.6/en/innodb-parameters.html

It's a performance optimization which dynamically builds hash indices to supplement the regular btree indices whenever MySQL notices a certain pattern of queries. Unfortunately, in 5.6 it takes a global lock when doing this which causes spikes like you're seeing.

You can turn it off by setting innodb_adaptive_hash_index to 0 in your db param groups. It's dynamically settable so you can experiment quite easily to see the effect with it on/off. If you upgrade to 5.7, you can tweak an additional parameter call innodb_adaptive_hash_index_parts which lets you split it up and avoid global locking.

You can also read more about it here https://www.percona.com/blog/2016/04/12/is-adaptive-hash-index-in-innodb-right-for-my-workload/