Psycho is famous for being a film which made the audience nervous about what it was going to experience as a result of some very short violent scenes, but very long suspense scenes. It is perhaps the ultimate example of Hitchcock's well-known explanation of suspense vs. shock:
Two men talking for five minutes. Suddenly a bomb goes off. A ten second shock after five minutes of boredom.
Take the exact same conversation but put a shot of the bomb ticking before it. The whole time the audience is thinking "they better stop talking or they're dead" etc. So you have five minutes of suspense.
Psycho fulfills this in a complex way by showing the audience the narrative-destroying potential (killing off the famous starlet) right at the beginning - just after they started to empathize with her. It is an approach far more radical and diabolical than any other filmmaker had tried previously. So the slow pace is essential to the suspense.
Just as the stupidest, most assinine "horror" films ever made are the ones being made right now, by idiots who want a "roller coaster ride" every second. A horror film MUST be slow paced, or it is no longer a horror film. It becomes a moronic action film with phony "vampires" (i.e. that wretched Tarantino travesty or John Carpenter's stupidest production) but utterly negates the entire aesthetic principle behind the horror film in its elemental form.
This is related to the music as well, because it is crucial to the development of these suspense scenes, and must also be very slow paced or even non-rhythmic. That is why rock music is so horrifically bad in a horror film. It cannot be fast paced or driving. It must be meandering things like irritating violin tremolos, soft flute solos, irrelevant percussion tappings and bumps, static choir harmonies, etc. - the usual stuff of horror scores but completely appropriate to the psychology of the form.
Herrmann's music in Psycho of course is the ultimate - the apotheosis of the horror film score. The atonal, bitonal, tritonal but deliberately "colorless" strings create a straightjacket of static madness punctuated by fits of hysteria around the entire film. Exactly as Herrmann stated he wished to do in his general approach to film scoring - to envelope the entire film in a single, unifying structure of sound. Only in this work of brutal genius, there is no escape from that structure.