Hello Ivaylo, the CPU usage and memory footprint you describe are normal. Our rendering engine comes from Firefox and is designed to run on much faster machines; Leopard Webkit (the only other more-or-less up-to-date browser for Power Macs these days) has a similar demand for RAM. So the choice is: Live with it, or use an older, more lightweight browser that's incapable to access most modern websites. Or buy a new Mac.
I can say that I use TFF on my PowerBook 1.67 GHz and it performs decently if you keep in mind that you're using a machine that's 14 years old. My PB has 2 GB of RAM installed, though, and an SSD instead of a spinning HD, and I would recommend this to everyone. I also use TFF on my 800 MHz G3 iBook, which has only 640 MB of RAM (its max. capacity), but also an SSD, so swapping is less of an issue. Finally I use it on my PowerBook G3 400 MHz (Pismo), which is very slow but still usable, it also has 1 GB of RAM, which helps a lot.