When you said the computer lcd blinks, it's a sign it's accessing your hard drive.
What programs were you running? What other background tasks are running? If background tasks such as scheduled virus scan, backup, updates are running while you try to do work or play any kinds of av on your pc, you will notice the slow down.
If you don't have background tasks running, it could be the programs take up lots of ram or accessing hugh amount of data from you harddrive or both, could even be a malware causes it. There are many reasons why a computer slows down, more detail would help.
B.