in order to build a "fast" computer, you have to eliminate
possible bottlenecks in your system.
In general your system will load and process faster
if you equip a cpu with multi cores and high clock speed.
However a outdated hard drive installed may cause significant
performance drop due to the cpu has to wait until data being transmitted to ram.
And it is hard to define a "fast" computer coz we all have different needs.
If you are a serious gamer, you may consider a comp wif multi core cpu, high ram capicity and high end display chips.
but if you don't demand that much and just a net surfer, you don't need to consider much as a 5 yr old comp can do the job right.