Making new account harder to create will only scare genuine new people, and it will not stop scammers and spammers.
Even now lot of people report that they can't create account until they pay evil fee, as they probable use shared IP address.
But adding some sign up quiz random questions can maybe slow spammers down
answering 3 Questions that need a little research may work.
or better yet 3 math problems.
add three 3 digit numbers
111+123+140 = 374
or four problems
add four 4 digit numbers
1001+1002+1003+1004 = 4010
Yeah have a script generate 4 random number tell the person add the 4.
once a correct answer is given have a 60 second clock count down to next set of four 4 digit numbers
This wouldn’t be difficult to implement, but it also would not be difficult to bypass via a bot parsing the numbers being displayed and solving the problem. Parsing handwritten numbers is considered the “hello world” of neural networks (machine learning models often used on images), and the bar to beat this countermeasure would be low.
The only real way to prevent bots from solving problems is to use a
Adversarial network that adds noise to images that the user needs to classify. This is difficult without knowing the model a potential adversary is using to classify the image via automation.
It is not so much that the numbers can be added quickly as it is the delay from
question 1 correct answer to question 2
correct answer to question 3
correct answer to question 4
the bot will have to wait 60 seconds to answer next question it can not speed the delay of 60 seconds.
so that is 3 minutes no matter what.
you could also have a hidden timer. say 5234 + 1284 + 7345 + 4567 = ? a real human would have a calculator and add the 4 numbers in about 10 seconds to 30 seconds the answer is 18430
So first question gives 4 numbers randomly and randomly assigns a time slot to give answer in 30 to 55 seconds
so first question pops up person adds 4 numbers and wait for time slot this is at least 30+ seconds.
then 1 minute timer for second question to appear. now we are up to 90+ seconds add random time slot we are past 2 minutes
wait 1 minute to see third question assign random time slot 3 minutes and 30=50 seconds more and 4 minutes time minimum has past.
wait 1 minute for last question up to 5 minutes assign random time slot and close to 6 minutes to do a free unknown account.
If you want to do things fast you can give money and a traceable confirmed email.
So fast sign up is money and verified email.
Slow unknown is play the adding and timing game.
A super human can be that fast. 
