Good idea but things would slow down tremendously if adding one builder would lower the time by only 20%.

The upgrade from X-Bow level 4 to X-Bow level 5 takes 10 days. I propose that adding one builder to an upgrade increases the upgrade speed to 200% speed, a third builder increases the upgrade speed to 300% speed, a fourth builder increases the upgrade speed to 400% speed, etc.

With four builders all working on four different X-Bow's to level 4, that would take 10 days to complete for all of them. With four builders working on one X-Bow, however, that would take 2 days and 12 hours. Multiply this by four, and that's also 10 days.

Now, let's apply your idea of how fast builders should work to this.

One builder would take 10 days on the X-Bow. Two builders would take 8 days on the X-Bow (80% of 10 days). Three builders would take 6.4 days (80% of 8 days). Four builders would take 5.12 days (80% of 6.4 days). Multiply 5.12 days by four, and that's 20.48 days. That's a little over twice the amount of time used then if you were to just upgrade all four X-Bow's using one builder each.

So yes, I think a 20% time reduction is a little bit too much at the cost of one builder. If you up this up to (x times 100%) speed, where x = amount of builders used on an upgrade, then I think it would be a great idea.