Oh I definitely read them, regardless of how easy or difficult the content was to understand.
So I would advise reading all passages.
However, how much time you spend reading depends on the difficulty level of the passage.
On my actual MCAT, I had two extremely difficult passages and I remember I took around 20-25 mins for them (combined, that is). I re-read them a couple of times to fully understand.
So ultimately how slow or fast you read them should depend on how quickly and how well you can understand the material presented in the passage. However, I would advise to read them all anyway.