PHP 5 - XSL Slow?

I've just installed PHP 5 under Fedora Core 2 and I've been having
speed problems with xsl and the dom functions. Previously I used
the sablotron extension functions under PHP 4 with script execution
times of under 5 seconds ( max of 0,2M xml files, with a fair
number of xsl:key statements). Under PHP 5 execution times almost
seem random and never seem to use the full system resources
(memory-limit wasn't included as a compile option) -- script
execution times are often ten times longer that under PHP 4. Has
anybody else encountered problems with these functions and/or is
there a way to tell PHP to stop pussy-footing around and use the
system resources available to it (this isn't a shared environment)?

Thanks for any help.