bug#29846: scroll and fontify takes longer time for file in slow network

[Shuguang Sun <shuguang@gmail.com>]

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Hi,

Windows 7, GNU Emacs 27.0.50 (build 1, x86_64-w64-mingw32) of 2017-12-09.

Issue:
I have remote dirver (//server/path) mapped to my windows system (samba?).
The network is slow, and it takes some time to open the file. However,
after I have the file opened and scroll down, it takes a long time to
scroll to the next screen. It gets worse if the jit-lock-debug-mode is on.
After that if I try to scroll back to those has been fontified, it get
fluent as expected.

If I open the same file in a local driver (speed is higher than the network
definitely), the scroll is very fluent.

If I open a txt file (fundmental-mode, not fontification) in the same
remote directory, the scroll actions are fluent as well.

Per my understanding, all actions should be done in memory when a file has
been read into buffer, except backup, autosave, etc. The response should
not depents on where the file located phically.

How to repeat it:
1. Find a slow transfer rate network (sorry, it may be hard)
2. Prepare a long c file, 3-4 screens.
3. take font-lock-mode on (jit-font-mode)
4. Open the c file and scroll down, scroll down, scroll down...
5. scroll up, scroll up, scroll up

I have tried profiler. It indicates, the jit-font and redisplay take
majority of the cpu time.
1. open the c file
2. profiler-start
3. scroll down, scroll down, scroll down...
4. profiler-report


Best Regards,
Shuguang Sun

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