How does it perform then comparing to BDBJE and JDBM?  Out of curiosity,
if there is a good file-based cache store implementation, why BDBJE and
JDBM backends were introduced?

Haven't tried BDBJE, but it certainly is faster than JDBM. We should setup Hudson's performance plugin 

http://wiki.hudson-ci.org/display/HUDSON/Performance+Plugin

so we can monitor regressions.

Tristan