GUI keyboard repeat rate suffers with many long lines

Steps to reproduce

1. Open MacVim (mvim --clean)
2. Create a line 80 characters long (80ii<Esc>)
3. Go to beginning of line (0)
4. Hold down 'l' and time how long it takes to get to the end of the line (for me, about 3 seconds)
5. Delete this line (dd)
6. Create a line 700 characters long (700ii<Esc>)
7. Make 90 copies of this line (yy90p)
8. Go to column one, row one (1G0)
9. Hold down l and time how long it takes to get to column 80 (for me, about 6 seconds)

(This affects insert repeat rate as well as cursor motion.)

(Also, I don't think this was a problem before the recent update to 9.0, but I could be wrong.)

Expected behaviour

The repeat rate shouldn't be affected by the data in the buffer.

Version of Vim and architecture

VIM - Vi IMproved 9.0 (2022 Jun 28, compiled Jul 25 2022 08:23:03) macOS version - x86_64 Included patches: 1-65

Environment

MacOS 11.6.4
MacVim GUI

How MacVim was installed

Internal update; originally installed from download

Logs and stack traces

No response

Vim configuration where issue is reproducable

No response

Issue has been tested with given configuration

• by running MacVim.app from GUI macOS interface
• by running vim/gvim/etc installed by MacVim
• by running other versions of vim (e.g. /usr/bin/vim)

Issue has been tested with no configuration

• by running mvim --clean (or gvim, supplied by MacVim distribution)
• by running vim --clean (in terminal, supplied by MacVim distribution)
• by running vim --clean (in terminal, other suppliers, e.g. /usr/bin/vim)

Other conditions

• The both Homebrew packages "vim" and "macvim" are installed

Start by reproducing the slowdown in MacVim with mvim --clean, using the short and repeated long-line steps from the report, and compare GUI behavior with the listed clean terminal Vim runs. Trace the GUI keyboard-repeat and cursor-motion path; done means repeat speed no longer depends on buffer contents for the reported cases.