Long duration '/deleteblocks' beyond mapgen_limit if mapgen_limit is reduced in map_meta.txt

[paramat]

Discovered the issue due to this report:

00:40 Out`Of`Control hi
00:40 Out`Of`Control anyone could give me example of '/deleteblocks (x1, y1, z1) (x2, y2, z2)'
00:40 Out`Of`Control as i get error
00:41 VanessaE       I only know `/deleteblocks here`  (and I think you can put a radius after it)
00:42 Out`Of`Control uhm
00:42 Out`Of`Control VanessaE: thanks
00:44 Out`Of`Control ok i delete blocks but they are still there
00:44 VanessaE       they probably re-generated.
00:45 Out`Of`Control its outside mapgen limit
00:46 Out`Of`Control area near does not regenerate
00:46 VanessaE       maybe the mapgen limit also trims the "delete" coords
00:46 Out`Of`Control so i need disable limit delete then enable?
00:47 Out`Of`Control its just one area far away

Investigating this, /deleteblocks actually worked but extremely slowly.

My test:
Start a world with normal mapgen limit.
Teleport to and generate some world at 0, 0, 10000.
Exit the game.
Change 'mapgen_limit' in 'map_meta.txt' to 1000.
Restart.
Use 'deleteblocks' to try to delete the now unwanted area of world at 0 0 10000.

Result:
For even a small volume of world such as 2x2x2 mapchunks '/deleteblocks' is successful but takes a very long time, with a 'map-sqlite.journal' flickering continuously in the world folder and a continuously busy harddrive.