Level Structure for Builders
Builders should have a leveling structure similar to troops and spells. This could be as simple as Level 1, Level 2, Level 3.
Level 2 builders would be able to build at twice the speed (8 day upgrades would take 4 days).
Level 3 builders would be able to build at three times the speed.
Assuming that there are only three levels, you could have level 2 builders be available at Town Hall 8 (when builds start taking 5+ days) and have level 3 builders be available at Town Hall 9 or 10 (when builds take 10+ days).
