This is a gigantic change we are working on for years, and is getting to the final stages. During it we have a period where “things will get worse before they get better”, and this is one of the “worse” side effects of it.
As soon as we complete New installs will default to Ember CLI builds in Production for all existing sites, and rip-off the old asset pipeline, we can then aggressively start modernizing it and hopefully collect some upstream gains.
There is also the potential for us to allow people on slow CPUs to opt-out of source maps and other “nice to have” features to speed up their rebuilds.