CPU Performance - Using Pixelmator

So I own a late 2007 MBP with a 2.2GHz C2D CPU, 2GB RAM and 128MB Video card.

I like to use pixelmator (photo editing program). Now for the most part I tend to run very few apps at the same time and if I do run a lot most are simple apps that don't take too much CPU power unless they are crashing.

But there come at times when I notice or usually always that my CPU is idling at around 70 to 90%, which I think means that 70 to 90% of its CPU is free, correct? (I am using iStat statistics btw).

the real problem is when I start to use some of the filters and more advanced features in Pixelmator, it tends to only sometimes start lagging even when there is a lot of free CPU power. Can anyone maybe suggest how to fix this or if this is more of an app problem?

Also if I am making a mistake with something, thanks for clarifying me.