[amelia] Expedite Overimputation

I'm trying to run overimputation as a diagnostic on a large data set with 20 multiple imputations from Amelia. The overimputation diagnostic on one variable takes a long time to complete (more than 2 weeks). Is there any way to expedite the overimputation procedure? For example, would using the subset argument to select a subset of cases expedite the procedure? If so, how would one use the subset argument? I tried using a vector of TRUEs/FALSEs to indicate whether to keep a row, but I received an error. Any suggestions for expediting overimpute and/or using the subset argument of overimpute() would be very helpful. Thanks! -Isaac