First off your running only 4GB of RAM, if your trying to run 20 or 30 tabs at the same time, plus streaming music, plus the AV running, etc,. your more then likely just low on RAM. Second to that, depends what that 750GB drive is, if it is a 5400RPM drive, then..... it.... takessss.... a loooongggg.. time to read the data / write the data (cache for your browser) and is a performance killer with the RAM, this especially true for Win8 which is very sensitive for that.
Secondly I hear what you said, but as we don't know your system much less what has happened to it please bare with us and just follow these basics please:
Download and run SPECCY, copy and paste the first tab to show your idle temps
Can Air dust out the bunnies and use paint brush on the vents, coolers, fans, etc. you can access externally.
Did you install all Windows Updates? Including OPTINALS except BING? Check them and repeat till ALL are installed.
Download and run Slim Drivers, install all the latest updates for alot of the other things 'under the hood' the video card interfaces with, but you don't need to reboot until you do the last update
Remove whatever AV your using and get AVIRA http
/www.avira.com/en/download/product/avira-free-ant... (or go to www.filehippo.com and download Comodo/Panda or AVG (AVAST! was hacked so I don't recommend them anymore)) and do a full system scan - this repeatedly has resolved alot of people issue relying on MS Essentials.
Download Malwarebytes do a full system scan (AV doesn't pick up alot of malware) - this resolved almost ALL other similar posts to date as most had Malware the AV didn't pick up.
Repeat the AV/Malware scans till the system comes up clean (( YES we heard you SAY it was clean, but just because you checked it for one thing or using something as poor as MSE, doesn't mean there isn't still Malware or things loading poorly in the browser)).
Go to Programs and uninstall ANY and all TOOLBARS listed.
Download Update Checker from www.filehippo.com and scan for then update all the stuff it finds. I can't count the number of times a old copy of JAVA or Flash gets 'stuck' or never updated and causing issues because the new 'browser update' is looking for the new 'code' but only finds the 'old code'.