There's some miscommunication here.

> How does eat detect a visual command in eshell?

eat-eshell-mode doesn't detect visual commands and launch a separate eat buffer, like eshell-visual-commands do. It filters all process output in eshell and handles term codes. It turns the eshell buffer itself into a terminal, so that vim or whatever runs in eshell.

> It sounds from your description like vterm is faster than eat.

vterm is faster than eat, but a dedicated eat buffer is fast enough for most common TUIs. An eshell buffer with eat-eshell-mode is slower.