I noted that you said you have already scanned for malware. Which anti-malware/anti-virus software are you using?
What type of hard disk do you use? If its NOT an SSD, is there any chance it could be heavily defragmented? Have you check that recently?
Can you re-post your Processes screen, but this time maximise it a bit more, and sort it by the most Memory used?
Lastly, please don't bother with installing 3rd party PC optimisation software : they cause far more trouble than they are worth.