The problem here is that the file has a very very long line which is slightly larger than 800 kilobytes, containing most of its content. This essentially freezes buffer:index_line. I have tried adding a fallback implementation which switches to vim.fn.matchstrpos on very long lines because that function has a start index argument and so doesn't require repeatedly slicing a string (but has to be used as a fallback because of the cost of calling into Vimscript functions, so on typical files it is considerably slower than vim.regex:match_str), but that doesn't seem to be much of an improvement, most likely because Lua strings have to be recopied as Vimscript strings. I think this is a good argument in favor of introducing an option like max_line_length which would set the max number of characters (not bytes since blindly slicing on bytes can create invalid UTF from multibyte characters) in a line which will be used by the indexer, and setting it to something like 40 kilobytes by default. I can make a PR.