Sounds like something is eating your memory or processor. Can you play music/videos normally, or do they slow down as well?
One thing to check the next time, is to right click the task bar at the bottom of your screen and select Task Manager from the menu that appears. Click the Processes tab and click the Show processes for all users. Something using up memory would show up in the Description column. if you don't have a Description column, click on View, select Columns to select what you want.
Click the Memory column heading to sort the numbers high to low. That will show at the top if anything is using lots of memory and slowing everything down. Clicking on the CPU column heading will also sort high to low to check for high CPU usage from something.
Capetonians reboot would cure it as you get a fresh start, as it were, but things would eventually return, so no real fix. The trojan that YPJT mentions is a possibility as something is doing something it shouldn't! Run a scan with Malwarebyte's Anti-Malware and/or Spybot, and let us know how it goes.
How many programmes do you have open at the same time? Running say, Outlook, several browser windows and a media player, and trying to type might just tip your comp over the edge; especially if you do not have a lot of memory installed.
Does it slow down with just the one programme running, or lots?