How to reproduce (link to view file):
curl -LO 'https://github.com/rust-lang/rust/raw/a6fe6c9be10c79d64c988d490519f5cc07e9060d/src/librustc_borrowck/borrowck/mir/dataflow/impls.rs'
time target/release/examples/synstats impls.rs
Running with syntect from master (currently 431174d), this is the output of time:
target/release/examples/synstats impls.rs 141.49s user 0.54s system 99% cpu 2:22.71 total
Sublime text build 3125 seems to have no problem with the file and loads instantly.