You forgot to add a description.txt.
Done.
How about taking this a step further, and only saving newly added/modified bricks? That would remove ~90% of any lag.
Then you'd have a bunch of partial saves with tons of floating bricks.
A script to combine them would either be run when you wanted to load them (maybe, but it'd add time) or on saving, defeating the purpose.
For now it only saves if a brick has been planted since the last save.