i believe protonm recommends that rsa 4096 bit keys should be generated on PC-desktop-class compute devices and not mobile phones 
one thing with protonm that people aren’t aware of is that the encryption key that the contact list is protected with is SEPARATE from the other keys and i haven’t found a way to change it from 2048 to anything more secure. i’m probably too paranoid in this case but it wouldn’t hurt for them to implement an easy way for people to change it should they wish to do so …
another point. encryption keys have EXPIRATION dates and they should be manually changed together with the password/passphrase at least once/year imo.
i’ve seen that some people working in support at Purism with keys that haven’t been changed since 2016 … perhaps they don’t need to but public keys are … well … PUBLIC, so for customers peace of mind they should not neglect this aspect …