LeaderF: Leaderf rg is much slower than using rg directly

• vim or neovim?
  • [ x] vim
  • neovim
• Output of vim --version or nvim --version: 8.1 431
• Output of :echo has("python3"): 1
• Output of :echo &pythonthreedll(only vim, not neovim): libpython3.5m.so.1.0
• Output of :py3 print(sys.version): 3.5.3 (default, Sep 27 2018, 17:25:39) GCC 6.3.0 20170516
• Operating system:
  • Linux
  • Mac OS X
  • Windows
  • Etc.
• Configurations related to LeaderF in vimrc:

let g:Lf_PreviewCode = 0
let g:Lf_UseVersionControlTool = 0
let g:Lf_DefaultMode = 'FullPath'
nnoremap fg :<C-u><C-R>=printf("Leaderf! rg --stayOpen -e ")<CR>

Steps to reproduce

$cd .vim
$time rg set

$vim
fg set<CR>

Actual behaviour

using rg directly rg set 0.84s user 4.71s system 37% cpu 14.650 total use leaderf rg more than three minutes, and make vim slower.

Expected behaviour

both of them spent almost the same time, vim is not stuck