Editing your own work is one of the hardest parts of writing. The trouble is, you know the story, so it's hard to tell whether you've given the reader insufficient detail or too much. Asking someone else to read it is always a good idea–preferably more than one person. Failing that, put it aside for a few weeks, if possible, so you can come back to it fresh.
    There are various 'rules' to stop your work being slow, which is usually taken to mean 'boring'. Generally, writers are told to cut descriptive passages to a minimum and avoid adjectives and adverbs, but there are plenty of exceptions. E. M. Forster starts A Passage to India with a description of the landscape. Readers who have bought the book or already heard of Forster would no doubt keep reading, even if the opening was slow (I'm not saying that it is. In fact, it's totally apt), but these days, if it arrived on a publisher's slushpile by an unknown author, would it have been dismissed after the first paragraph? Whether to cut descriptions or pare down sentences depends on what they add to the whole piece. Do they create an atmosphere e.g. foreboding, suppressed passion etc.?
    One way I've stumbled upon to delete unnecessary words is to apply the Tweet principle. I was looking through a manuscript to find Tweet length extracts. I discovered sub-clauses that could go, two similes where one was enough–you get the picture. Of course, there's a danger of cutting off too much flesh, so I recommend keeping your original manuscript until you've had a cooling off period, so you can restore any edits that have gone too far. At the end of all this, if you're still unsure, you could to worse than to remember: if in doubt, leave it out. Unless you happen to be E. M. Forster, less is likely to be more.