It's memory usage will probably be higher but it shouldn't be any more of a load on cpu or disk than pushpool... My testing has shown it performs quite a bit faster in most scenarios and most of those tests were very heavily loaded. Admittedly I was quite liberal with the amount of memory available though limiting it's memory should impact it's performance by at most a few tens of % not by orders of magnitude.
My best guess would be memory usage. psj can eat as much memory as you let it and by default it's pretty greedy. But it can also run very lean. By default with the -server switch the default max heap size is something like 1/4 or 1/3 available memory so my guess would be you've got virtual mem disk paging happening.
Try setting the java switches -Xms16m -Xmx64m
That should be enough headroom for a pool that size. Remember java stores all it's work recorded internally and doesn't offload to memcached so it's guaranteed to eat more than the pushpoold process if you run them side by side.
Also are you running a 64bit OS? If so I'd highly recommend using a 32bit JDK. The 64bit JDK only really offers the possible advantage of using heaps larger than 2gb. If you don't have that requirement all it offers you is near double memory usage and often slower performance. If a 32bit JDK is not an option then you can use the -XX:+UseCompressedOops switch to reduce it's memory usage at the cost of small performance hit.
If you've already tweaked memory then it might be the other way around... if Xmx is too low the JVM will spend all it's time trying to free memory.
If you think I'm off the mark perhaps you could send me the properties file and I'll see if anything stands out...
One other thing to consider is because of the JIT compiler psj actually needs some time to 'warm up' before it reaches peak performance. On a flat out out server this can take a minute or two, on a less loaded server it could take longer though in that case I'd be surprised if the difference was noticeable. You can force it to precompile a lot faster with -Xbatch switch.
Here's a good explanation of some of the more common JVM tweaking params:
http://download.oracle.com/javase/1,5.0 ... /java.html
(it's for java 1.5 but it's all still relevent)
and if you want get really hardcore about it try some of these:
http://www.oracle.com/technetwork/java/ ... 40102.html