ezfilter.vim

function! s:fuzzy(lhs, rhs) abort
  return a:lhs =~ join(map(split(a:rhs, '\zs'), "printf('[\\x%02x].*', char2nr(v:val))"), '')
endfunction

let g:asyncomplete_preprocessor =
  \ [function('asyncomplete#preprocessor#ezfilter#filter')]

let g:asyncomplete#preprocessor#ezfilter#config = {}
let g:asyncomplete#preprocessor#ezfilter#config['*'] =
  \ {ctx, items -> filter(items, 's:fuzzy(v:val.word, ctx.base) != 0')}

Try this instead so it does ranking.

if has('python3')
  " osa_filter is slow when running in vim script so only set if python3 is enabled
  let g:asyncomplete_preprocessor =
    \ [function('asyncomplete#preprocessor#ezfilter#filter')]
  let g:asyncomplete#preprocessor#ezfilter#config = {
      \ '*': {ctx, items -> ctx. osa_filter(items, 1)},
      \ }
endif

I wish vim script gained lshift() function which accelerated osa_filter. The algorithms are different in python and vim script versions.