I was using a third-party plugin for markdown (I think it was called pandoc.vim), but for LaTeX files I was using only plugins that come with Vim. The documentation in :help tex-slow did suggest things to put in .vimrc to help make syntax-highlighting faster and I did try all of them. The only thing that solved the lag was disabling syntax highlighting for LaTeX completely.
The slowness I experienced for LaTeX files happened even without any third-party plugins installed, using a one-line .vimrc that only turned on syntax highlighting. So I think it is unfair to say "the slowness has nothing to do with Vim itself". Probably also "or the low power of the netbook" is unjustified, in the sense that the tips in :help tex-slow do likely solve the problem on computers a little beefier than my old netbook (which is probably more than 15 years old at this point). I mean, those suggestions are in the official Vim documentation presumably because they did work for someone.
Think of it this way: if the slowness of LaTeX syntax-highlighting were not a problem in Vim itself (where by "Vim itself" I'm including the vimscript files that ship with Vim, not just the executable), would it be documented in the official Vim documentation?
The slowness I experienced for LaTeX files happened even without any third-party plugins installed, using a one-line .vimrc that only turned on syntax highlighting. So I think it is unfair to say "the slowness has nothing to do with Vim itself". Probably also "or the low power of the netbook" is unjustified, in the sense that the tips in :help tex-slow do likely solve the problem on computers a little beefier than my old netbook (which is probably more than 15 years old at this point). I mean, those suggestions are in the official Vim documentation presumably because they did work for someone.
Think of it this way: if the slowness of LaTeX syntax-highlighting were not a problem in Vim itself (where by "Vim itself" I'm including the vimscript files that ship with Vim, not just the executable), would it be documented in the official Vim documentation?
https://vimhelp.org/syntax.txt.html#tex-slow