> This checks out.  I tested previews with lualatex for a bit, and it's
> about 8-10x slower than pdflatex.  One big chunk of this gap is the
> absence of precompilation, the rest appears to be inherent to lualatex.
> I'm not familiar with lualatex, so if someone knows how it can be sped
> up I'd be glad to be educated.  For now I'd recommend using pdflatex for
> LaTeX previews.

Yes, you're right. Tested pdflatex myself and the difference is like
night and day. My choice to use lualatex was based in the fact that I
write in greek as well and sometimes may need greek script in latex
fragments. But, seeing the difference in speed, both in buffer
previews and in live ones, I'll probably stick with pdflatex and just
work around the language part.

> I've had trouble getting LaTeX environments to show via eldoc in the
> echo-area, whereas eldoc-box has worked fine.  I'm making a note to
> investigate this later.

In my setup it only works if I open the eldoc buffer. But eldoc-box
doesn't work.

>> I tried setting `org-latex-preview-default-process' to `dvipng' and
>> found that previews as well appear blank. Don't know what might be the
>> problem.
>
> Is this in the context of previews or HTML exports?

Both. But with pdflatex it works.