It doesn't do the flattening thing anymore, although now as the progress level rises weird lines appear at the 'meniscus' and clip outside the circle and disappear.
And by slower I actually meant exponentially, although I notice now that it wasn't just getting exponentially slower.
Here's what's actually happening when I try to make it match the build time:
10 seconds build time, animation finishes prematurely.
20 seconds build time, animation doesn't finish in time.
30 seconds build time, animation finishes prematurely.
50 seconds build time, animation finishes prematurely.
60 seconds build time, doesn't animate at all (or extremely slowly).
I tried replacing your progressbar with JesusHipster's version but I ran into the same problems.
This leads me to believe that the formula I'm using might be faulty. The build times themselves are working properly because I checked them alongside a unit with an expiration timer and the latter died as the buildings finished construction so it must either be a problem with the formula or the animation speed trigger doesn't function properly.
PS: A team-coloured alternative is always desirable. You could make it a portrait version and let people chose which one they prefer.