Please relate to This thread
It is also advised that you change from Creative mode to Survival Mode if you are in such mode, as it severely increases the world loading time.
It's been noted that shift-clicking items into the Incubator(And causing a crash due to a bug from that reason) will make your world take forever to load in the future. Just avoid shift clicking items into anything maybe except chests as there might be bugs waiting to crash your client.
As of now there's no known fixes for the slow loading, sorry.
Also the shift clicking could be a major factor as last time, I played for a long time & created a new storage area which I transferd everything by shift clicking. It used to only take about 5 minutes.
Im having the same problem... It takes about 5-10 mins to load up mah main world...
And whats with the singleplayer aether "Server lag" thing? It's like in multiplayer since i can break and place blocks but chat does not work or commands.