First of all, it's an old slow processor, you're not going to change that no matter what you do.
Second, if you have a 32 bit OS installed, then about 3.5GB of RAM is all you're going to be able to use, period, but that's probably more than the system can utilize anyhow, so it really doesn't matter. You're CPU limited here, not memory limited, unless you're trying to run more things at once than the system can handle on the best of days.
Third, I'd download HWinfo or another utility and check your thermal/temperature readings. Sounds like maybe the unit is either getting hot and throttling the CPU speed or you have an infection. Make sure you have both a good antivirus and a good malware scanner installed, and run full system scans with both.
If the system is clean, then you're just stuck with the fact that the CPU performance you have with that unit is about equal to what was common six or seven years ago, and it isn't going to keep up well if you're multitasking or running demanding software.