document how to minimize fold computation costs

[Konfekt]

See the discussion at lervag/vimtex#3054

[lervag]

I'm curious, on line 85, it says: "The most efficient is to call a compiled function without arguments:" - however, in my experimentation I really did not observe any noticable difference. I'm curious if anyone actually did, or if this is just based on someones hunch?

Perhaps we should also add a line above line 105 saying something like: The result values "=", "s" and "a" are more expensive, please see |fold-expr-slow|.; it may make the new section more discoverable.

[Konfekt]

I'm curious, on line 85, it says: "The most efficient is to call a compiled function without arguments:" - however, in my experimentation I really did not observe any noticable difference. I'm curious if anyone actually did, or if this is just based on someones hunch?

It links to

#  runtime/doc/vim9.txt (lines 1441-1453)
Calling a function in an expr option ~
							*expr-option-function*
The value of a few options, such as 'foldexpr', is an expression that is
evaluated to get a value.  The evaluation can have quite a bit of overhead.
One way to minimize the overhead, and also to keep the option value very
simple, is to define a compiled function and set the option to call it
without arguments.  Example: >
	vim9script
	def MyFoldFunc(): any
	   ... compute fold level for line v:lnum
	   return level
	enddef
	set foldexpr=s:MyFoldFunc()

which makes sense as soon as one realizes that Bram added this file after implementing Vim9script.
I now rewrote it to make this clearer at the end of the slow section

[lervag]

Ok, I think this looks quite good now. 👍🏻

[chrisbra]

thanks!