Fast blur is marked as Legacy in CC 2018. What's the best (and/or fastest) equivalent to use now?
Gaussian Blur is now GPU accelerated, and it's the one that they suggest as the successor to fast blur. They've even included a script to update old projects to use the new blur, look for it in file>Scripts>UpgradeLegacyBlurs.

If you want one that uses the same maths as fast blur, i.e. is the most visually similar, then I think Box Blur is the one.

The official announcement is here.