Many things can be the problem for you. I'll list what I know from my knowledge which may or may not help your issues.
If you run your computer a lot for a long time, it may start to over heat, like all computers they have a fan. And if the fan begins over heating, it signals the computer, and the computer slows down. - If this is the case, you can put a cooling pad on the computer, or a fan. If you have neither. You can shut your computer off and let it sit for a couple hours, or even just turn it off before bed. Completely off, not in sleep mode. Just off.
Depending on the type of computer you have, in my case windows 7. You can go to Control Panel, and look for Hardware & Sound. From there you can go to Power Options. There will be 2 different choices, or an alternative choice. If you want to help over heating issues, go with "Power Saver". Otherwise just choose Balanced.
If you already haven't, you can clean your computer. Now there is several hundreds of ways to clean a computer. This is how I clean mine.
First I open all my browsers or the latest ones I use. First thing I do is clean out all the cache and cookies I use for gaming and such. You can find that at this link. http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html#117717 From here click on the little box "Delete all sites". While you remain on that link, You will see a bunch of icons on the box. The top left second. It looks like an earth with an open folder and an arrow pointing down. Click that. Then click and drag the bar to the furthest right to "unlimited"
---- This will speed up game play that you play with adobe flash, mainly facebook games.
After said above is done, click your browsers settings, and clean it out, History, cache, cookies, saved forms and passwords, and downloads. Get rid of that junk it's a waste of space. Okay! Close the browser
Now you can go to your start menu and search for "Disk cleanup" and click and run. After it calculates, you click every box. and click Ok.
That will remove extra junk, even cleans your recycle bin.
Okay so now that you completely cleaned the crappy stuff off your computer, it's time to do the easy part. You have 1 of 2 choices here.
You can either reboot. Or shut down. I prefer shut down. Shutting it down will give it time to calm down. Rebooting is like taking an angry person and pulling him away from a fight and throwing him back in the fight after 5 seconds.
So that's about it. I hope it helps.

Edit - 
XP remembers negative (DNS unable to resolve) entries for a few minutes. Often this can be a pain when moving and debugging sites. As long as that negative entry sits in your DNS cache, your box will continue to give you a cannot find server error. By cleaning out your DNS cache, you can fix this problem. Rebooting will flush your cache… but there is an easier way.

Go to start, type in "command prompt". Click. Then type - " ipconfig /flushdns " without the "". Type that correctly just like that, ipconfig /flushdns and it will instantly send a command to flush dns. Problem solved. 

This works for Windows Vista, Windows XP, Windows 7 & 8. 

Well, I never knew that. I thought it was just the internet. I will try to use your advice. It seems like it is a great advice. Thank you so much for the information. ♥

Black Bot :  
Well yes the internet speed will also greatly affect speed in general. You may notice a slow ping sometimes if you have bad connection, but mainly posted above is all tips and ways to clean to speed up browsers and games. (: You're welcome. 

Wow, I never knew that. I always thought it was because of that dumb Mobogenie thing I installed (and uninstalled), but I stand corrected. Thank you for the advice !

Hello.
Hey, I have another way to speed up computer. Computer run slowly because of too much undeleted temporary file in your temp folder.
Besides, virus is easy to be found in those temp folder. So you need to remove them manually (if you know how) or just download a Temp File Cleaner (Easy to be handled). This?Temp File Cleaner is always free and it has small size but, need latest JAVA application (recommended) and?NET Framework 4.0 or higher. If you want more detailed information, ask me a little or simply just search on Internet. I work with computer carefully for a long time so, it never slows down.

Angel_Raphael90 :

Oy yes!~ I've been using Temp File Cleaner  V 4.2 and it is really comfortable and easy. It does improve my PC speed and reduce its memory space. It doesn't make any damage/change  to my PC system unlike those troublemaker (Cleaner... Trial program).