Finding fault with the imaams of the masjid

There's a few brothers in a masjid who usually don't like when any scholars from different localities come to their masjid, when jamaat comes and even their own Imaams. They find fault in them. For example, they say that one person is reading too fast, another is making ruku and sajdah too long and they only come when the Imaam has to go on the mimbar for the khutbah.

Will their Salaah be accepted like how they don't appreciate their Imaams? Please advise as to what the normal brothers can do and also what the Imaams can do.