Ctrl-[ is the same keycode as escape in terminals, and it's much easier to reach. For me, remapping jk in insert mode results in lag after typing 'j'.
If you want a plugin that makes it so that the j appears immediately regardless of whether you're trying to invoke the keymap or not, you can use something like https://github.com/nvim-zh/better-escape.vim
It only creates lag if you actually want to type 'jk'. Otherwise, you keep typing like normal, and it doesn't matter. You should be able to config the delay, too. I use `fd`, and I can count the number I've noticed it.
That little lag on the j doesn't matter, when you notice you're already a few keys ahead anyway.