The system performance concerns many aspects, not just memory allocation. This memory allocation is just the way that linux OS does. And it is proved to be relatively efficient.
Your case is different.
I do not know what web browser you are using. I guess probably it is Mozilla/Netscape. Actually one of the biggest problem in mozilla development is the startup speed is very slow. And the web browser is a big resource consumer. So if your linux box is not customized very well. Then this high demand browser may cause some problems.
And, after you start the machine for a while, it becomes slow. Do you ever try to figure out what application takes up the resources? More information should be collected, or we are just guessing the error. There may be some applications take up too much resources(CPU usage for example.) e.g. by default, there are several system maintenance applications running, such as updatedb ... Those applications are running in the backgroud, which will consume your system resource also.If you just open your web browser at that time. Then the problem may occur.
The video card should be considered, too. Is the driver the latest one? Is the video card function correctly? Some physical problems can also cause the system running slow, or even hang. e.g. the OpenGL part of my video card got problem, and I am using the openGL driver to do the X displaying. Thus, the X server will often hang due to the hardware problem. I replace the driver. Then everything is ok.
Also, are you using font server? those font render is a big consumer. Try to configure the box without those font renders, they will make the system very slow and not stable.
.......
Actually your machine is much better than mine. Since my linux box is functioning quite ok, yours should be ok also .
－
－
－