With that idea, you (the captcha maker) would also have to write some code that computes how long humans should take to do a task (so that you can time the user and compare that with what your code spits out). Whatever code you write, the bot makers could eventually figure out what you wrote, and copy that.
To put it another way, when you say “humans would spend more time on the second task” with your two examples, you would have to write specific rules about how long humans would take, so that your captcha can enforce those rules. But then the bot makers could use trial and error to figure out what your rules were and then write code that waits exactly as long as you’re expecting.
With that idea, you (the captcha maker) would also have to write some code that computes how long humans should take to do a task (so that you can time the user and compare that with what your code spits out). Whatever code you write, the bot makers could eventually figure out what you wrote, and copy that.
To put it another way, when you say “humans would spend more time on the second task” with your two examples, you would have to write specific rules about how long humans would take, so that your captcha can enforce those rules. But then the bot makers could use trial and error to figure out what your rules were and then write code that waits exactly as long as you’re expecting.