Why Is My Computer Running So Slow

• Have you cleaned/deleted your temporary files/history/etc recently?
• Are you updating your OS and drivers regularly?
• are your plug-ins up to date?
• Is your security system catching all spyware/malware, viruses - they can really slow you down?
• Is your computer overheating? Does it need a physical cleaning and perhaps a professional one?  Is the fan working properly?
• Have you tried defragging? I know some folks don't like this one but it might be needed.
• Do you have too many add-ons? or are you running too many programs at start-up?
• Your computer may be running out of hard disk space
• corrupt files and programs can cause slowdown
• are you leaving your computer on for days on end?
• is your internet speed fast enough - do you need to upgrade?

Anyway you asked why but since we're not there, you might hae to begin by making sure that you have updated, cleaned, cleared and run various scans.