:Files is un-naturally slow the first time

- [x] I have fzf 0.23.0 or above
- [x] I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
- [x] I have read through https://github.com/junegunn/fzf/blob/master/README-VIM.md
- [x] I have read through the manual page of fzf (`man fzf`)
- [x] I have searched through the existing issues

Off late, (for the past 2 weeks), **the `:Files` command takes much longer than usual to appear for the first time**. I am using `fzf 0.27.2 (e086f0b)` and `neovim v0.4.4`. Searching after the pop-up window appears is fast, subsequent invocations of `:Files` is fast. The problem occurs only when I open Neovim for the first time and invoke `:Files`.
I have benchmarked my SSD, and its performance seems to be about the same (its also a new one, only about 6 months old).
Also, fzf works just as fast as usual when I do history search in the fish shell.

Reproduce the delay with the minimal Neovim configuration command included in the issue, comparing the first and subsequent :Files invocations. Read the fzf.vim and fzf README files referenced in the report and trace the :Files startup path; done means identifying and correcting the cause of the first-use slowdown without regressing later searches.

Written by the indexing model from the issue text.