Back in 2010 Matt Cutts from Google said speed would be considered for rankings... at that point not a big percentage, but nonetheless it is important, and increasingly so. Many Page Speed insights recommendations are easy to address with htaccess Gzip but others would be difficult to maintain after each update to a Muse site overwriting them.
Developers make it very easy to add a few plugins in a CMS (like joomla or wordpress) and get additional combining and compression for javascript and CSS... hopefully Muse developers will be able to do the same. This really is becoming important and I hope it starts to see some attention from you (also responsiveness and non-BC blogs.. have to always throw that in especially after reading what Andrew Hushbeck said)