Enabling the thread pool leads to about 5% performance loss in MariaDB 10.0 and 10.1, but not in MariaDB 5.5. I tested 5.5.49 vs. 10.0.25 vs. 10.1.14.
The benchmark is sysbench OLTP read-only with 1000 point-selects per transaction. The benchmark machine has 16 cores (32 hyperthreads).
See attached spread sheet for numbers.