[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.