Is my RAM compatible with my Macbook Pro I have a Mid 2012 13-inch MacBook Pro. To speed it up, I bought  two Kingston PC3L-12800 1600 MHz memory modules which have 4 GB each. However, my computer keeps showing the famous "beach ball" if try to do to many things at the same time (keeping multiple tabs open, closing applications one after another etc).

Do you think there is a compatibility issue? If so, is there a way that I can solve it?
Yes, there's a problem:

PC3L ≠ PC3.

PC3L is PC3 Low Voltage memory that operates at 1.35V and PC3 operates at 1.5V.  The 2012 MacBook Pro calls for PC3-12800.  See Everymac.com.  For best results you want to use the exact memory, or higher quality memory that operates at both voltage levels.

Also, that Mac is capable of 16GB of RAM, and the price point is low enough that you could max it out for only a marginal amount more.  Nobody ever said "Gee I have too much memory!" It's better to have and not need than need and not have!