The CAPTCHA tax
Every puzzle you put in front of a visitor costs you some of them. Here is what it costs, who pays it, and what to do instead.
A CAPTCHA is a tax on the people you want to hear from, paid to keep out the people you do not. Like most taxes, it is easy to add, hard to remove, and nobody measures what it costs.
Who pays
Not the bots, mostly. Solving services charge a fraction of a cent per puzzle and headless browsers handle the checkbox variants. The visitors who pay are the ones with slow connections, older devices, screen readers, non-Latin keyboards, or simply less patience: a real person who wanted a quote and left when asked to find the traffic lights.
Published estimates of the drop-off vary widely, and the honest answer is that it depends on the audience and the puzzle. But on a contact form the arithmetic is unforgiving: a form that gets twenty real inquiries a month and loses one to a puzzle has lost five percent of its leads to stop spam that would have taken thirty seconds to delete.
What it is really protecting against
Two things: the volume of junk, and the occasional dangerous submission (a phishing link, a fake invoice). Both are better handled after the fact by something that reads the submission than before the fact by something that interrogates the visitor.
The alternative
Make the visitor do nothing. Catch the bots with signals they cannot see: an invisible field they fill in because they fill in everything, the time between page load and submit, a sender’s history across many sites. Then read the message. Deliver what is real, hold what is doubtful with a reason attached, and give the site owner a button to correct you.
That is what spamex does, and the visible part of it on your form is nothing at all. Real customers never know it is there. The only people who notice are the ones whose messages stopped arriving, and they are not customers.
If you must keep one
Put it after a failed check, not before every visitor. Show the puzzle only to submissions that look automated. Most people will never see it, and the ones who do are exactly the ones you meant to slow down.