PR summary

FT2Font currently re-reads data from the font file on every glyph load, costing 10 lseek and 4 read syscalls per glyph on every FT2Font.set_text call. This is still relatively fast on a local filesystem, but imposes huge IO strain on remote filesystems (WSL's /mnt/c bridge to windows in my particular case, but also network mounts).

This PR mmaps the font file and hands the buffer to FreeType, so glyph loads skip the Python layer and only touch the filesystem once. Python still handles opening the file for unicode path handling, with a test added for that.

Note that we build freetype with meson's auto_features=disabled, so its mmap=auto line was a noop and removed in this PR. It doesn't matter here since we're handing freetype the data instead of a path.

Measuring syscalls directly on the font file for 30 glyphs across 10 set_text calls, we drop from ~2900 lseeks and ~1200 reads to 1 lseek and 1 mmap. When using the local linux filesystem on my machine, set_text is ~15x faster and a savefig on an empty plot is ~1.5x faster. When I cross the WSL filesystem boundary, the speedups are ~1000x for set_text and ~10x for savefig.

import matplotlib.pyplot as plt
for _ in range(50):
    fig, ax = plt.subplots()
    fig.savefig("/dev/null", format="png")
    plt.close(fig)

Before (Across WSL filesystem boundary):

After:

AI Disclosure

Discovered myself, investigated and prototyped with claude code, manually reviewed / edited.

PR checklist

• [n/a] "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• [n/a] New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines