[6502nerdface]

The input sequence would be <C-v>10jI#<ESC> (I've just added escape at the end).

I'm guessing the delay you are referring to is the delay imposed by your terminal emulator (not vim) to detect the Escape key at the end that terminates visual block mode. If I'm right, you can quickly press "j" just after you press ESC, which should cancel the ESC-detection delay and cause your comment characters to appear instantly.

Edit: there's actually a few things that could be at play, including your terminal emulator, your screen/tmux (which is like another terminal editor, really), and vim's timeoutlen and ttimeoutlen settings.

[randlet]

This is interesting! For me using <ESC> does indeed take a few seconds to complete commenting out the block. However, I have jk mapped to <ESC> and the commenting happens instantly using jk which makes sense in the context of your explanation.

[hsitz]

It is interesting, I just tried pressing 'j' immediately after <esc> and the block prefixing does happen right when I press 'j'. The usual lag is just a second or two for me, though, using Gnome Terminal on Ubuntu. Also, there is no lag at all for me in gvim, even without pressing <esc>j.

[philh]

I wouldn't expect the terminal emulator or screen/tmux to cause a 5s delay in this situation; and if they did, I'd expect them to cause it in neovim as well as in vim.

[cyphar]

This does happen with neovim too. In fact, I had this issue in neovim even though I hadn't had it in vim. It was a one-line config change in my tmux.conf to fix it though, so no skin off my nose.

[ramblenode]

You can also add this to your .vimrc:

  set noesckeys

[thomasahle]

This worked! Sweet! Maybe neovim just has this as default..