It is the masterpiece of film. On a rewatch it is definitely the masterpiece. The shot sequencing and pacing is impeccable. The film experiments to perfection with deep focus photography, creating aesthetic alignments of subjects in the frame to serve the purpose of creating three dimensionality while still being presented compositionally as if it were a painting. The camerawork is forward thinking, kind of like Hitchcock, moving it around to translate the psychological progress of the character through the camera angles.
I will say at the beginning of the film there's a moment of a nurse passing the broken rosebud on the ground that feels very much like a student film experiment from today. Shooting through the object in the foreground in an almost forced way but it's well done. It was for the purpose of exaggerating the emotional and psychological experience of death.
THIS IS A CULMINATION POINT OF FILM FORM. As the 1920’s experimented with the removal of theater from film, i.e. using the close up and associative editing to focus on specific moments and sequences rather than just the actors crossing frame, and it was perfected throughout the 1930’s with lots of movement in terms of tracking shots, we've come to a height of progress of experimentation with the camerawork that yields Citizen Kane. Every frame is heavily mired in camera and lighting trickery from the streams of light pouring down into the room of the library where the Reporter goes through the archives and the camera passing through the doors with him. To the camera moving into the restaurant where his ex-girlfriend is being interviewed from above, looking in through the glass and then almost passing through it down to their conversation, Welles takes every opportunity to mess around with his camera to create meaning and explain the psychological conditions of his characters.
In terms of the performances though I will say you're seeing a shift back towards Theater based acting. There's a lot of exaggeration onscreen and scenework between actors that feels more reminiscent of watching a stage play than what was common film acting at the time. There's very little fast talking banter. The actors all take their time with their beats, walking around rooms, half-drunk, half-asleep, observing the others around them, listening. It's a very stark contrast to the hyper-active, charismatic, highly enunciated form of acting that we know from Cary Grant, Jimmy Stewart, Clark Gable etc...
Orson Welles was a theater actor and brought a primarily theater based acting troupe on to Citizen Kane so it makes sense that the style of performance marks a break from what had been established through the 1930's as traditional onscreen acting that was a bit more hyperactive. And honestly that style of acting came from the silent film actors who had to exaggerate everything so it would be crystal clear to the audience what was going on without sound whereas in theater you could still hear the actors so they could be a bit more naturalistic. You can already see glimpses of what's to come with Elia Kazan and Sidney Lumet and the directors of the 1950’s and 60’s who brought theater as well back into cinematic performance and the beats too of the edits.
Interesting fact: Bernard Hermann scored the film suggesting another parallel with Hitchcock in terms of pioneering new film form.
Orson really launched a new movement of cinematic technique based off true experimentation and forward thinking, thinking that was intentionally uninhibited by the conventions of the day.