If the user solves the CAPTCHA in 0.0001 seconds, they're definitely a bot.
If the user keeps solving every CAPTCHA in exactly 2.0000 seconds, each time makes it increasingly likely that they're a bot.
If the user sets the CAPTCHA entry's input.value property directly instead of firing individual key press events with keycodes, they're probably either a bot, copy-pasting the solution, or using some kind of non-standard keyboard (maybe accessibility software?).
Basically, even if the CAPTCHA service already has a decent idea of whether the user is a bot, forcing them to solve a CAPTCHA gives the service more data to work with and increases the barrier of entry for bot makers.
I found several websites switched to 'press here until the timer runs out', probably they are doing the checks while the user is holding their mouse pressed, it would be trivial to bypass the long press by itself with automated mouse clickers.
