This is probably a symptom of something that's going to screw us when we start deploying it as a server.
How are you sure the PNG and QNG take so much CPU time ?
When I've made a few calls, I get spikes on my cpu graph (applet in gnome-pannel) around the time png/qng show up in the logs, and top says it's butterfly that's responsible. Attaching a profiler to a running program is kinda tricky though, if you don't want to get your figures distorted by the activities that happen before the problem arrises.
It probably won't become a massive issue ever, because when you time-average it, the spikes only take up <5% of my computing power.
It's probably going to be something really subtle like accidentally triggering a full garbage collection sweep. If you're not up for investigating it, I'll understand.
Great, but this sounds quite hand-wavy.
David, please re-open when you have more information regarding the actual problem.