I think I have mentioned this request long ago but have not created a topic to ask for this feature. Using the Extract Table Spec node and converting the displayed type to the data type which can be passed as a flow variable to other nodes (e.g. GroupBy) slows down the workflow.
I have to pass column types to the GroupBy node to determine the type of aggregation columns since they may change. (for example Int or Double). So I use the Extract Table Spec node to find the types and then change them to the type that can be passed to the GroupBy node (e.g. Double to org.knime.core.data.def.DoubleCell).