Just to report back, the slowdown was because we were specifying a nodeModules entry (epsagon) in defaultFunctionProps. Frank pointed out that this means a package.json is created for each function with that package in it and npm install run for each function which was what was causing the long build times. As it turns out it's no longer necessary to do this for Epsagon (it was when we started using it) so our solution was to remove the nodeModules entry altogether. However, if you have something specified there, be aware that builds will take a bit longer.