mapExtFunc on dataset consumes time

[josiahsams]

Invoking mapExtFunc API on Dataset's takes considerable amount of time. This values gets aggravated when the API is used in an iterations specially for ML kind of workloads.

mapExtFunc API typical does a lazy operation and triggers the transformation it suppose to do when an action API is invoked later. So the invocation should be in few milliseconds.

But the current the API takes nearly 1.5 second (1626 ms) on each invocation.