Basically, the more difficult and illegible the captcha, the higher the drop in conversions. If you don't believe it, experiment on your site with captchas with varying difficulty and take notes of the results.

Standard captchas that require the user to fill in letters and numbers are the most acceptable of all but they still are quite of a hurdle. Audio and video captchas are a real kill because they might require up to a minute of your users' time to listen to or watch and fill in. This is why it's hardly surprising that audio and video captchas are the ones with the highest give up rates.

Unfortunately, no matter how much technology advances, the real captcha alternatives are yet to be seen. This doesn't mean you are completely helpless against spam. Here are 3 captcha alternatives you might want to try:

Use a less annoying captcha variation. Out of the numerous captcha variations, there are some that are more acceptable to users, for instance the variation where a user has to solve a simple math problem (like 3+4) and enter the result. This captcha variety eliminates the illegible letters that piss users most and if you use simple math problems all your users can solve, this might solve the captcha problem for you.