The performance of mysql fulltext depend on order of keyword
https://dev.mysql.com/doc/refman/5.6/en/fulltext-fine-tuning.html#fulltext-optimize

set GLOBAL innodb_optimize_fulltext_only=ON;
OPTIMIZE TABLE `table`;


Please run that, time your variations again, and report back.

If it fails to help, report a bug at bugs.mysql.com .