Arasan 15.5 is available from http://www.arasanchess.org.
Changes in this release:
1) Fix bug that can cause illegal move from pondering.
2) Check for checkmate before returning 50-move draw.
3) Send Winboard variants in feature line (only "normal" is supported).
The first of these fixes is for a regression introduced in 15.4.
There should be no performance difference from previous versions, unless you are using pondering, in which case losses due to the illegal move bug may show up in 15.4.
--Jon