Poor performance with large files

[pamburus]

I am experiencing extremely poor performance with large assembly source files.
It takes 4 minutes for bat to process a 45-MiB assembly source file.

⇛ time bat target/release/deps/bench-f36188efc4eb690a.s --color always >/dev/null
bat target/release/deps/bench-f36188efc4eb690a.s --color always > /dev/null  237.31s user 10.16s system 102% cpu 4:00.49 total

⇛ lsd -lah target/release/deps/bench-f36188efc4eb690a.s
.rw-r--r-- pamburus staff 45 MB Sat Feb  8 18:30:04 2025 target/release/deps/bench-f36188efc4eb690a.s

⇛ sysctl machdep.cpu
machdep.cpu.cores_per_package: 10
machdep.cpu.core_count: 10
machdep.cpu.logical_per_package: 10
machdep.cpu.thread_count: 10
machdep.cpu.brand_string: Apple M1 Max

The average processing speed seems to be about 190 KiB per second.

[keith-hall]

Related: #304 (comment)

[pamburus]

Yes, this issue seems to be related, but the reasoning about performance bottlenecks in #304 does not seem to match reality.

Here is the reality:

About 92% of the time is spent in syntect::easy::HighlightLines::highlight_line, which in turn spends about 97% of that time in parsing.

Only about 3% of the time is spent in write_fmt, so buffering the output at this point will make any visible difference.

It looks like the performance problem needs to be redirected to syntect. It will probably not be easy to fix.

[pamburus]

As an experiment, I tried replacing the output writer with std::io::sink.
Surprisingly, this reduced the total processing time by 13% instead of 3%, but the bottleneck remains the same.