I noticed today that there was a very strong correlation between quantity of screen covered by a nebula vs. the lag experienced.

Perhapse nebulae need to be pre-rendered like the planet-shadows & the paralax backdrop?

george moromisato 4 Apr 2015:

It wouldn't hurt to render them on multiple threads.

george moromisato 24 May 2015:

Fixed in 1.6 Beta 5. We now use multiple threads to paint nebulae.