Vim slow at rendering the buffer with LaTeX syntax highlighting

Question

After fixing a hard-drive failure, I am installing GVIM on my Thinkpad X230T laptop running Windows 10. Things are working well except for the rending speed of buffers for *.tex files. The problem is: with syntax on, the buffer of latex renders exceptionally slow. Here goes a list of "features":

1. It takes seconds to refresh a buffer of size 1920*1080 (in pixels) when I press <C-f>;
2. If I shrink the size of the buffer, the rendering gets faster. Still, I shall experience a second lag at minimum. During the lag, I can only see a blank buffer.
3. Text operations are also slow. Simply starting a new line below by pressing o shall also end up with a one-second-lag. This is largely due to the rendering of the whole buffer after adding that single line: all the lines that follows shall be refreshed for their "new" position.

As sample screencast:

I have applied all the suggested options mentioned in this post (https://stackoverflow.com/questions/8300982/vim-slow-running-latex-files), yet I still get the same slow rendering speed.

I was only experiencing this exceptional slowdown on my X230T. With exactly the same _vimrc on the other two desktop machines, I don't even experience a tiny lag in the rendering speed. Nor did I experience the rendering problem with previous installation of Windows 7 and 8 on the same X230T laptop.

Lastly, for hard drive, I am having a SSD on my X230T, which worked pretty well so far; and I have HDD on both the other two desktop machines.

Any advice would be very helpful!

Thanks a lot!

All the best,

-Linfeng

Answer 1

The problem is, vim's regular expression engine is really slow, and I guess latex is pretty demanding on regex.

I was able to get vim a bit faster by doing:

:syn clear texSectionFold
:syn clear texPreamble

These were the main offending syntax group regexes.

I found them by doing:

:syntime on

Now press ctrl+L a ton of times and wait for vim to catch up. Then:

:syntime report

This gave the following output:

  TOTAL      COUNT  MATCH   SLOWEST     AVERAGE   NAME               PATTERN
  8.903872   1911   52      0.017387    0.004659  texSectionFold     \v%(%(\\begin\{document\}.*$\n)@<=^|\\section)
  4.979438   1859   0       0.016382    0.002679  texPreamble        \v%(\\documentclass)@=
  0.634906   1976   182     0.010863    0.000321  texEnvName         \v%(\\%(begin|end)\{)@<=\a+\*?\ze\}
  0.373173   1859   0       0.000880    0.000201  texArgsEnvNormReq  \v(\\begin\{%(theorem|lemma|proposition|corollary|conjecture|definition|remark|example|proof)\*?\}\s*)@<=\{
  0.317732   1859   0       0.000468    0.000171  texArgsEnvNormOpt  \v(\\begin\{%(theorem|lemma|proposition|corollary|conjecture|definition|remark|example|proof)\*?\}\s*)@<=\[
  0.223595   1859   0       0.000341    0.000120  texDimen           \v-?%(\.[0-9]+|([0-9]+(\.[0-9]+)?))%(pt|pc|bp|in|cm|mm|dd|cc|sp|ex|em)>
...

So you can see that those two regexes are way more expensive than others. I don't use folds, so I had no issue disabling that. Not sure why finding the preamble is so slow.

Perhaps this helps you too, @llinfeng. I hope so!

EDIT:

To run those syn clear commands when vim starts, you would need to put them in ~/.vim/after/syntax/tex.vim. It won't work in an ftplugin file, as the syntax definitions are not yet loaded at that point.

EDIT2:

Turns out some of the syntax groups I posted above came from a plugin. Disabling the plugin doesn't help, as then some other syntax groups are slow. Eesh!

My final fix, and all I ask from syntax highlight, is:

$ cat ~/.vim/after/syntax/tex.vim 
syn clear
syn match texComment   "%.*$"                                            
hi def link texComment  Comment

It's really fast.

Answer 2

You have used vim -u NONE filename.tex when opening the file to disable your .vimrc and other scripts from loading for the session, and this has fixed the problem.

The conclusion is that something started by the .vimrc file causes the slow-down.

Try to selectively delete parts of the file to find out what exactly is the cause. Remember to take a backup.

Answer 3

Partial Solution for Thinkapd X230T per se

Problem Identification

As @Edd has suggested, use :syntime on and syntime report to identify the most time-consuming syntax-highlighting "group/region/match"';

Partial Solution

As to be found in the following URL, I have loaded the tex.vim in the after directory of my Vim distribution (c:\vim\vimfiles\after\syntax\ for my case):

https://github.com/llinfeng/Vim/blob/X230T/vimfiles/after/syntax/tex.vim

Outcome

Using the same LaTeX file, my improved performance goes as follows

Open question --- how to replicate the syntax-highlighting effeciently

The syntax coloring is not ideal, as compared to the native tex.vim to be found as c:\vim\vim80\syntax\tex.vim.