I load up the game both the Mojang and Minecraft screens pop up. The menu screen then comes up and you can see the underwater background and it works fine for the first 10-20 seconds. Then everything slows down making the whole interface and my PC unusable. It will then make my whole computer slow down. If I'm quick enough I can enter one of my worlds and play or play with someone else.

The game itself when it's in a world works great however if I'm not fast enough it'll just slow down and crash.

It takes anywhere from 2-5 minutes to completely crash.

 
I've been having this problem with the game ever since the Aquatic update.

NOTE: I also have this problem and am forced to lower AA down to 1. 

I have a Toshiba Satellite A505-S6965

CPU: Core 2 Duo P7350

RAM: 4 GB

GPU: ATI Mobility Radeon HD 4570 512 MB dedicated memory + Shared memory