From top, free and cat/proc/cpu info

You have a Pentium M 1.6 ghz which runs at 600 mhz, so processor is throttling
like it should Pentium M is a Pentium Mobile, are you using a Laptop or a
desktop (with miniboard?)

You have 1 gb of Ram.

Your Pluginwrap of opera is causing a problem, you've tried another browser
(which one?) and it gives you the same problem.

Am I in the ballpark here?

I'm running a much smaller version, also Debian, a Toshiba Laptop with only 512
mb 1 ghz proc Pentium M and I have no problems. (Iceweasel with fluxbox)

So back to basics. Put in a default Firefox and see what it does, deinstall
Opera for the time being.

Also, what kind of video card do you have?