c-ts-mode is slow with large buffers.

[Alan Mackenzie]

Hello, Emacs.

Just for fun, I downloaded the feature/tree-sitter branch, and tried
scrolling through a large C buffer, from file packet-rrc.c.  This file
is part of Wireshark, and was the topic of bug #45248.  It is just under
10 MB big.

To scroll all the way through the file with c-ts-mode took ~53 minutes.
For comparison, in C Mode it to ~157 seconds.  That's a factor of around
20 difference.

Also of interest, towards the end of the buffer in c-ts-mode, the
font-locking gave up completely.  It was in an initialisation for a big
brace block.

I'm probably not saying anything people don't already know, but the
measured timings might be helpful.

-- 
Alan Mackenzie (Nuremberg, Germany).