More Effective Coroutines (MEC) is an improved implementation of coroutines.
When you use Unity's built-in default coroutines you will have to be very careful about how you use them or else they will allocate memory that the garbage collector later has to clean up. This can cause your app to randomly skip frames. MEC solves that issue for coroutines, while also making coroutines more powerful by running about twice as fast as Unity's default coroutines do and providing you with a host of new options that Unity's default coroutines don't have.
This is the Pro version. The free version of MEC contains the mininum set of features, whereas the Pro version contains the full set. Both versions run on the same super-fast core.
Switching between MEC coroutines and Unity's default coroutines is really easy. They can even be used side by side in the same project.
