Starting with Flash Player 18.0.0.203, a  noticeable performance lag can be seen with Audior when encoding to MP3 format on Mac OS systems in all browsers.

The issue itself has to do with crippling performance on Alchemy libraries. Alchemy lets you use C/C++ code in Flash.  Audior uses one such library, the Shine Alchemy MP3 encoder.

A whole thread dedicated to this issue can be found on the Adobe forums where you can find updates on the progress of solving the issue.

Bug reports have already been created by other users that have been hit by this serious issue:

We suggest you go to the links above and upvote the reports so that we can speed up the process of solving it.

The release notes for the Flash Player 19 beta update are promising. The Known Issues sections mentions this particular issue. You can read the full notes here.