In mistune through 2.0.2, support of inline markup is implemented by using regular expressions that can involve a high amount of backtracking on certain edge cases. This behavior is commonly named catastrophic backtracking.
https://github.com/lepture/mistune/releases
https://github.com/lepture/mistune/commit/a6d43215132fe4f3d93f8d7e90ba83b16a0838b2