The old adage was that "total page weight" and "number of assets" were the big drivers of slow web sites.

HTTP/2 should make "number of assets" less of a problem, since it no longer drives multiple connections, assuming all are from the same site/domain.

Edit: Bloated JS and parse time might be the right contender to replace "number of assets" if you want to point to the top 2 current issues.

It also prevents you from using immutable caching.