This type of captcha has effectively reduced spammers from ~200 per day to ~zero per day. We will not be removing the captcha. It takes a lot longer than 30-40 seconds to identify and delete 200 spam accounts and all their posts every day. That is time taken away from developing and maintaining a solid product for you and other paying customers. If you want to blame anyone for this inconvenience, blame the spammers.
Most people complete our captcha in about 10 seconds. If it takes longer, it might be during a period where a lot of spammers are trying to get in. During those times the captcha requirements are automatically increased. If you are aware of other stats about our web site, such as "new commers [who] leave at first step", perhaps discussed on another site, I invite you to share that link.
For comparison, I have been to sites where it takes almost 2 minutes to complete a captcha.