By deleting some downloads. Some downloads can cause virus which can cause your computer to be slow. Also, delete some useless application/apps which can help form lagging. Lastly, don't install any download that can't be trusted.
You can preform maintenance checks on your computer that helps most cases. 2.Shut down programs that you aren't using(helps with macs) 3.Sometimes you just need to restart your computer to get a fresh start with your computer.
I hope this helps because I have the same problems :D
