The Goodison Park (built 1892) away dressing rooms are really Spartan and uncomfortable. They are poorly heated, are right under the stand (noisy), there's only one toilet etc.

I don't think Everton's home record can really get that much worse when they move into their new stadium next season though.

Some of the All or Nothing documentaries that cover Premier League teams include a lot of footage in the away dressing rooms, and they are almost all bad (though Goodison was weirdly cavernous and looks more annoying than normal).

Exactly how they are bad changes, though - when you take the Emirates Stadium (Arsenal's home ground in London) tour, for instance, they actually include some details about how the table in the middle of the away dressing room is designed to be uncomfortably high in a way that keeps team members from making eye contact, which is something that the stadium designers thought would be annoying. At one point, at least, the self-guided tour narration actually included a comment that Pep Guardiola hated the layout.

I toured Stanford Bridge in the middle 2010s, they actually found uncomfortable away dressing rooms weren't the most effective as it either got the other team to go out and start warming up or riled them up.

When Mourinho started he brought in a sports psychologist who made the dressing rooms slightly heated with light pink walls and a comforting atmosphere. They ended up going 2+ years without a loss at home after that.