Why aren't you on Neovim yet?

Why not? Because I don't understand the configuration, to be honest. I've tried it and it's too much like magic, and the things that are supposed to be great about it fall into precisely three camps:

1. Stuff that's also in Vim since version 8.blah now.
2. Stuff that I'll never use.
3. Stuff that doesn't work.

So it's a learning curve for no benefit. I might try again sometime I have a weekend to play around.

EDIT: I just installed it, aliased it to vim and used your configuration snippet (though my .vimrc is actually ~/.vim/vimrc but that's a niggle).

• Exactly like VIM, but faster
• Better performances

That has not been my experience. I have seen micro-benchmarks of LuaJIT outperforming vimscript, but in practice, neovim was noticeably slower on larger files. I've spent days trying to identify the root cause and disabled or removed many lua-based plugins (e.g. tree-sitter, lsp, lualine, ...etc) and was able to narrow it down to neovim's syntax parser. I've tried ruling out unoptimized colorschemes (esp. ones that have a lot of function calls to set colors), syntax files due to inefficient regex pattern matching, :syntax sync minlines=256 and :set synmaxcol=1500 but it all came down to :syntax off. Disabling it brought neovim's (v0.7.0) performance on par with syntax-enabled Vim (8.2.5050).

• Works out-of-the-box with my extensive VIM configuration
• Installation and configuration was a breeze, don't need to change anything

The gap will widen over time, esp. that Vim is coming out with vim9script and neovim has been working on making lua a first-class configuration language. More plugins are coming out now requiring luarocks dependencies.

why aren't you on Neovim yet?

I was on Neovim since 0.4 (~2019) until 0.6 (~2022) and I switched back to Vim.
I was using Neovim because it was interchangeable with Vim, but Neovim ecosystem is moving at a frantic pace with everything being re-written in Lua and supporting Neovim-only APIs. I believe (I hope I am wrong) Vim's upcoming vim9script will be the final nail in the coffin of the interop story between Vim and Neovim (ref).

Considering:

1. how daunting it is to implement custom functionality in Lua and have to interface with vimscript through Lua APIs [0]
2. the breaking changes in Neovim APIs between 0.5 and 0.7
3. plugins requiring v0.7+, which in turn would require me to build neovim from source on virtually every remote host I may need neovim on because v0.7 is not available in the package manager repositories,

I would much rather use Vim and write Vimscript, even if vim9script wasn't a thing, just because of how stable Vim has been and I don't feel like I am missing anything in Vim-land, besides some eye-candy which isn't very portable anyway.

It almost feels like NeoVim's decision to maintain interop with Vim at this point is forcing it to be in this awkward middle-of-the-road position, where it tries to be as close as possible to Vim but different enough than Vim. Instead, a clean break from Vim would allow Neovim to design and implement clean/easy/pleasant APIs that are more suitable for their future direction, than trying to maintain compatibility with Vim that is getting more tenuous with every release.

0: Compare this contrived lua:

vim.api.nvim_create_user_command(
  'Foo', 
  'call some#vimscript#function(<args>)',
  { 
    nargs = '*', 
    complete = function(A,L,P) 
      vim.tbl_filter(function(value) 
        vim.... ??? I just want to filter results like `v:val =~ a:A` but WHY IS THIS SO HARD!!! 
      end, vim.fn.systemlist('some shell command')) 
    end
  }
)

to its vimscript counterpart:

function! CommandCompletion(A,L,P) abort
  return filter(systemlist('some shell command'), 'v:val =~ a:A')
endfunction

command! -nargs=* -complete=customlist,CommandCompletion MyCommand
  \ call some#vimscript#function(<args>)

Who said I ain't running neovim 😁.

Because I use mostly default vim and choose to optimize other things.

Vim is enough fast for me , and short )