Large grid of building placements lags

There is already a BAR [issue](https://github.com/beyond-all-reason/Beyond-All-Reason/issues/2589) for this but I believe it is an engine optimization problem. When you use shift+alt to make a large grid of buildings the game goes to 1fps.

Start by reading the linked BAR issue 2589 and reproducing the reported case in RecoilEngine: use shift+alt to place a large grid of buildings while observing frame rate. Trace the engine-side building-placement path to identify the source of the slowdown; done means large grids no longer reduce the game to about 1 FPS.

Written by the indexing model from the issue text.