How expensive are MVars ?

I'm planning to implement a small OO language on top of GHC (think JavaScript) and need to decide on how to implement the mutable object graph that is required. The two approaches I'm considering are: - something on top of Data.Graph - using MVars as the object references. The MVar approach is the most appealing, since it would also allow the OO language to contain threads. How expensive is an MVar access (in GHC), compared to the graph navigation that would be required to resolve a reference using Data.Graph ? I know this is a fairly nebulous question, but any comments or suggestions are appreciated. --Nick Main