Unfortunately, xsl:for-each-group does not seem to support the 
saxon:threads attribute. Can anyone think of a workaround to split the 
processing of groups among multiple threads without causing a lot of 
overhead?

If there is no such workaround, wouldn't it be a nice (and cheap?) 
feature to add saxon:threads support to xsl:for-each-group? 
I think such a feature would in many applications allow for a better 
control over the thread-overhead/work ratio when using multiple 
threads.

Marc