If you 2011 macbook pro is slow, freezes a lot, it could be because it needs more RAM. They come with 4GB RAM but if you open a lot of browser window and doing multiple things, you will quickly find that 4GB is not enough the OS will start paging the memory contents to external hard drive. Here is the usage below
There are a couple of things look for. Free Memory is 520 MB. The most important thing that makes your computer slow is the swap used. Every time the OS uses swap, it is going to freeze. You can prove it by running activity monitor on. Work in you laptop as you would regularly do. As soon as it freezes, check your activity monitor. You will notice that the swap is either increasing or increased. This proves the freeze is because of memory swap.
There is an interesting fact about macbook pro (and other computers) freezing. Even when it freezes it does not uses full memory power. That means the freeze is because of other bottleneck not the CPU itself. In this case , it basically reads/write to hard drive and since the hard drive is quite slow, it has to stop on everything while fetching and swapping that data. This makes your laptop slow.
If you think this is the reason your mac is slow, because let me know in comments. Ideally the swap size should be 0 or close to 0. A possible solution is to upgrade you ram to 8 GB or 16 GB. Since RAM is really cheap now a days, it cost less than $50 to upgrade to 8GB. Note, the apple site list macbook pro upgradable to 8GB but you can actually upgrade it to 16GB. This is undocumented.