There we go. Your set up is a really low end set up mate. Your 3GB RAM is why the speed is falling. Your processor is a low end processor. So is your graphics card.
You need more space, for starters. You have to increase your RAM.
If you can get a new processor and graphics card, go for it. But increase the RAM first. And lower the settings to medium, not high. High strains the comp and therefore lower speeds