The computers slow down for THESE reasons principally.
The first cause is never do maintenance (cleaning, fixing, antivirus, defrag and whatever) and
2second is, more important, the software and operation system. These ones evolve, requiring more resources and performance. If you doing an update process maybe your pc does not meet minimum requirements.
Change a harddisk for a solidstatedrive and increase ram could help to maintain performance long-term.
The third is related to your Windows Registry if you are a Windows user (reference: Welcome to The Registry Library) or your Preferences Folder if you're a Mac user (reference: Preferences folder). Viruses and spyware can add, change and destroy files in these places. Installing and uninstalling programs (even perfectly safe ones!) can damage existing files or leave useless entries (called orphans), all of which can waste your operating speed.
The FOURTH is if you have viruses or spyware, which can occur when you install third party programs. One of the things they do is run in the background and waste RAM and processing speed.
Do all these and Perform speed here ScanmySpeed.comonce it will sounds better ...