Neovim is still a lot faster for me. A simple example is always commenting out a block <C-v>10jI#, which takes 5 seconds or so in vim (including 8.0), but is instant in neovim.
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.
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.
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.
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.
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.
Kill region, save, run/compile/read, undo if needed. With undo tree (or if you add commit to the process above) there is no need to comment just one region to turn off something.
Optionally, convert the region into a separate function/method and just don't call it while checking.
Commented code (as in, working code commented out, not comments in code) eventually rots/becomes cruft that at some point in time is going to bite you (or someone else in your team, or some maintainer in the future)
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.