Space+alt+shift command too expensive if there are blockers

Using space+alt+shift build command require cause lag if it's used on spaces with build blockers (terrain or units).
https://www.youtube.com/watch?v=fPlBvX3EmHc (1000 cons)
UPD: https://www.youtube.com/watch?v=tuK4rAWfCwo (50 cons)

Reproduce the space+alt+shift build command with build blockers such as terrain or units, using the two linked videos as behavioral references. Measure the lag for the reported blocker cases and compare the 1000-cons and 50-cons examples; done means the command no longer causes the reported performance problem.

Written by the indexing model from the issue text.