one of the reasons a computer can be slow is that you have installed service packs and updates. the up dates don't make it go faster. the updates make it go slower.
another reason is that there are too many applications installed, not enough RAM memory, multiple mismatched security applications.
another reason is that you have a single core processor. the single cores don't have enough cache memory to handle a lot of the newer demands and streaming, playing video, etc...
another may be that you do not have enough free space on a hard drive...whatever the reason if you have an older single core machine, it's probably a good excuse to rebuild.