I too just imported about 3300 blocks into a file I cannot share in V7, and got really poor performance (10-15fps). Performance especially tanked when I selected one of the huge block instances which showed the wireframe for all included objects.
However, saving the file as a V6 file instead, and then opening it in V6 made performance again smooth, even when selecting the blocks.
(Interestingly, the file went from 280mb to 400mb going from V7 to V6…)
Sorry for the not so useful data point… I also got light render bugs in V7 in that same file that do not appear in V6:
