The result returned by replSetGetStatus contains enough information to compute a "repllag" value for each member with an optime. Having the server actually perform this computation would avoid the need for it to be done (potentially poorly) client-side.

Presumably the most sensible definition of repllag would be latest_optime - optime, where latest_optime is the largest optime.getTime() value of any replset member (that has an optime), but anything that is self-consistent would be fine.