well, even if she was very fast, there would have to be a secret underground tunnel connecting the two houses, so that she could arrive in jud's house and go upstairs before he entered the house.
jud heard a noise from upstairs, went there, looked around and got attacked by ellie.
you may argue, maybe the noise wasn't from ellie, maybe the cat was up there (how did it get in?) and made the noise.
maybe jud forgot to lock the door (even though he knew she was after him?) and while he was looking around, she sneaked upstairs.
but that's too many "maybes" for me. also, it doesn't explain how she appeared back in her own house again, right on time before her mom arrives.
The cat likely got in through an upstairs window, the same way it got into Ellie's room earlier. Ellie then came in while Judd was upstairs investigating (she wasn't upstairs at all, she was at the bottom where she stabbed his heel).
i checked the scene again. you're right, it was obviously the cat which made the noise (by smashing a picture onto the floor) and ellie appeard at the bottom of the stairs... BUT before that, we actually see jud locking the door of his house! if ellie broke in by smashing a window or something, sure that wouldn't be left unnoticed (it would certainly be louder than the noise the cat was making).
and how did she manage do sneak out and in of her own house, unnoticed by her father? (apparently not through the front door).
another thing that bugged me: when ellie was startled by jud at the pet cemetary and fell, you'd think she hurt her leg because she just fell... but no, her leg was suddenly hurt because of a bee sting?!