Update statement with a function

I'm just curious. I was trying to optimize an update statement with a function

Ex

update circle set area = fnGetArea(radius) where id = 1

This update went 5-10mins and when I removed the fnGetArea, it went 5-10 seconds!

How did that happen...?

I want to google it but I'm not sure what to type in.

Please enlighten me! :D

Thanks in advance