Try to get the performance profiler reading from from designer studio-system-performance while refreshing the dashboard. It could be because you db interaction is taking some time or because of some complicated logic while fetching the data . You can use defer load as well.
Generally to read the unexposed properties from the blob column, it might cause performance delay depends on the number of unexposed properties that are being used in the grid. You can try to expose those properties and check the difference in performance.