Hi @einarmar,
Alembic is not always fast to read
The most common cases when this might happen:
1) Mesh resolution is too high
2) Network reading bottle neck
After you open Maya scene with rig ( no Alembic ), it is fully loaded to your RAM and it doesn't read scene file anymore
So e.g. if you remove Maya scene file from disk then current session will be fine and you still can work with loaded scene
But Alembic is reading data from file every frame so that might create a bottleneck
To improve alembic file size:
In alembic export options
1) Use No Normals
2) uncheck Write UVs ( if you don't need them )
Hope this helps !