Don't confuse speed with reliability, warranty and known limitations. Many have give good advice, so I am going to add additional considerations. Please consider the following as it may or may not apply in your case.
A computer is only as fast as the tightest bottleneck, slowest component. Example, if you are surfing the web and happen upon a site that is very slow to load but others are fast then chances are the site has a speed issue, not your computer.
If you are doing online gaming and constantly dying because it happened before you saw it coming you probably are experiencing some sort of lag.
Many people want a fast computer because of the internet usage. Might I suggest it is tough to answer your question specifically without you 1st providing specific objectives, conditions, specs, use-cases, etc. to better understand what problem you are really wanting to solve.
Things affecting net speed might include:
- WiFi (its cool, it always sucks as a known 'contention' due to how it works in general). Interference and signaling and much more also play a major role
- Operating System (yes, the choice matters). Linux will run circles around MS windows all day and night WRT network speed. Linux may never become so well developed without major changes in industry to compete with MS Windows on the same plane. They are different and both are necessary evils it seems.
- Bad browser cache (clear it occasionally and restart browser)
- Poor browser (IE for example is slow and IMHO royally sux. It just so happens to be what comes with your computer and what many people use). MS has been 'forcing' their browser on users for years and at atleast one point was sued for that technique.
- Cable internet? While they offer some of the fastest throughputs in speed, it is a SHARED connection. Your speed comes and goes. Online gaming does not behave nicely always with continuously varying speeds. Sometimes its so fast you dont notice the issue, but subscribe to a fully dedicated syncronous line and try that on you will witness different and fast like never before.
- Software updates, malware reach-outs and many other background services. What, you have 'live' wallpaper? Then you are sharing your speed with your evolving desktop images.
- Add-ons (browser or OS)
Please keep in mind that your computer (if powered on and connected to full-time providers) can use the internet whether you see anything on your screen or not. This is because your email applications can check mail in the background without ever opening up the client executable that shows you the pretty interface. This is because of DLL's (dynamic linking libraries) which like other library files are shared code. The email app that does the sending and receiving may generally be embedded in executable DLL's and not the EXE you see. So, with these and other apps communicating in the background, say byebye to speed.
- Anti virus software: its best to turn on most of its capabilities but the trade-off is SPEED. It cant protect you if disabled so you need to find compensation for this. Either thru temporary setting-changes, different program altogether, disabled while playing (strong discouraged), etc etc. Sames goes for FIREWALL.
- ISP provided equipment (modem/router/combo, etc). I gained significant speeds by sending my equipment back to provider, solicited known compatible and supported devices, separated modem, router and switch into separate purpose build devices and nearly doubled my 'usable' bandwidth. Available B/W doesn't change but what is usable and HOW, really matters.
SSD? or Spindle Hard Drives? it matters. SSD's are NOT always the fastest devices for computers unless you setup some type of RAID with many of them. SSD's have bottlenecks too. I would recommend to learn more about storage to decide what type, how many and what goes on which type, for speed. To get true performance, there are specialty disks out there (SSD) that can't be compared to walmart specials as walmart would never sell anything that good IMO.
Everything hardware needs to be fast, compatible and compatible between other pieces of hardware. Example: Motherboard might suggest a video card but that might not work so well with soundcard of particular model. Even thought they work with Windows or motherboard, that inter-operability is equally important. Software matters too, not just the drivers.
How many people just click Install, Next, Finish, thus accepting all defaults and also silently installs unwanted-ware. Always choose CUSTOM install. If you know what to change, do it. If you don't know, leave the box checked/unchecked by default but at least you get a choice.
At the end of the day, it pays to do your homework on hardware, software, service provider, networking specs and then build your own system that is what you want it to be. It is not necessary and some argue buying pre-made on a shelf is better. All I will say to that is you get what you pay for. If its on a shelf with a black cover on it you can't see whats inside or cant make out the name/model due to covers and stickers, you probably are paying too much for something that isn't what its being marketed to 'say'.