To assess if this is really only the redraw lightable issue you may want to try this branch:
https://github.com/TurboGit/darktable/tree/faster-lighttable
Note that this is not a proper fix (has some redraw issue IIRC) and it won’t be merged. But if testing this code you do not see anymore the sluggishness of the lighttable then we are talking about the same issue.