When they're in the cinema near the beginning and he sees the girl in the yellow dress…presumably - from his reaction - she's not walking towards him menacingly like 'it' does every other time in the film. Why not? Every other instance of it following them has it walking steadily towards them. Here though, he's neither panicked nor in a rush, and it's not obvious what it is (he only realises what it is when the girl he's with can't see the girl in the yellow dress), and then he decides to leave. You'd think he'd be used to recognising it/evading it though, as you get the impression it's been after him for a while.
I always thought it moved at the same speed, but when it's in the house, the girl in the kitchen is walking/staggering slowly, and then upstairs, the tall guy is moving fast as gooseberry fool towards the bedroom.
There's always at least one moment in a horror film when the main characters do something stupid, and in this it was when they'd set up the plan to electrocute it in the swimming pool. Clearly things don't quite go to plan, but when they shoot it and it's in the water, why do they just stand around and be like "uhh, is it dead?". Who cares, now's the perfect time to execute the plan!