Glimmer 2.0, which is in active development now, will deal with initial render performance. Whereas, Glimmer 1.0 (landed in Ember 1.13) mainly delt with re-render performance, but inadvertently slowed initial render performance. Not sure when Glimmer 2.0 will land, sounds like possibly in the next couple 2.x.0 releases, so 6-12ish weeks until beta (or more).
I can’t speak to optimizations for large lists, I haven’t had to deal with that just yet, so I’ll let others speak up on that topic. My only feedback would be to specify a key in the {{each}} helper.