First of all, this idea is based on a misconception:
"But most ransomware use the hybrid encryption approach and during that they make mistakes allowing security researchers to build decryptors."
This is my opinion is not correct. The first ransomware used just symmetric encryption. Now if that symmetric key is left then it may be possible to retrieve it again. You must make a terrible mistake in the implementation if you use hybrid cryptography with separate symmetric keys per file. Now there will of course still be a ransomware nitwit (these are not high payed professionals) that still managed to screw it up, but I don't believe that that is common.
I have searched the Internet looking for a comparison between speeds of RSA versus AES for bulk data encryption in modern computers but found nothing.
Oh dear, you must up your Google-fu.
The other answer mentions encryption speed, but there is more to it than that. I think that the encryption speed of RSA is not such a problem that needs to be a huge problem. However, there is another disadvantage: efficiency when it comes to size.
If you encrypt a plaintext using RSA then you will have a certain disadvantage when it comes to ciphertext expansion compared to the plaintext. How big the ciphertext expansion is depends on the padding scheme used. For RSA PKCS#1 v1.5 padding you can go as low as an overhead of 11 bytes (although larger padding sizes are more secure). For OAEP, well, somebody created a table here. So if you e.g. use RSA PKCS#1 v1.5 with a 2048 bit key you'd expand the ciphertext with 11 bytes per 256 - 11 = 245 bytes. That means that files will suddenly be a lot larger (something that might be very noticable) or you might of course run out of disk space to perform the encryption at all.
Another problem is that RSA requires the use of a random number generator to generate the padding (well, unless deterministic scheme is used, but that would be overestimating the prowess of the ransomware creator). If you keep hitting the random number generator then it may decide to request additional entropy, and that means that your system may run out of it. When it runs out, it will stall or the speed will likely degrade to a crawl. That will be rather noticeable, especially if there are other applications that request random data from the system.
Finally, although the encryption speed may still be enough to encrypt lots of data, it is questionable if the decryption speed will be sufficient. Decryption speed is much lower for RSA than encryption speed. That said, the person or persons behind the ransomware will probably not care all that much.