I face the issue that one of the Senior developers and actually one of the first employees of the company is regularly speaking bad over new-joiners. First of all it's important to understand, that we make all recruitment decisions together, to actually get the buy-in of every employee for a new colleague. Unfortunately, it has repeated now several times that the mentioned senior developer speaks bad about them in terms of their knowledge, speed and motivation after several weeks.
It's of course true that the new employees are not as fast as the people who're already in the company for 2 or 3 years, that's normal and they need time even with a proper onboarding to understand the whole system and also feel safe enough to do critical changes.
How would you handle this situation in terms of the senior developer, but also how to support the new employees better as it seems like they don't get enough support from the senior developer, based on their feedback.