Things that speed the pace up:
Easy vocabulary that most readers will know
Lots of physical action
Lots of short snatches of dialogue
Things that slow the pace down:
Wider vocabulary that may have a reader puzzling over meaning
Reflective writing (ie a character thinking things through)
You can make some slow writing fast, for example, by including lots of movement in your description - so 'flowers lined the path' becomes 'flowers bobbed in the breeze along the path'. And fast, action packed writing can be slowed down by writing out every physical action - a bit like seeing it in slow-motion on a film.
Non-stop fast, and you risk giving the reader motion sickness. Non-stop slow, and you risk the writer dropping off to sleep. Quite what the proportions are will depend on both your preference and your sort of writing - for example, a thriller is generally faster paced than a romance, but there are slow burning thrillers and breathless romantic romps.
Either way, mix it up. So, several fast scenes should be followed by a slower scene - perhaps a lot of action followed by a short scene of recap or reflection on the action. This gives the reader time to breathe and gather their thoughts. Similarly, several slower scenes need to be pepped up with a fast scene, to move the story along.
Think of the pace as a series of peaks (fast scenes) and troughs (slow scenes) making a wiggly line which gets tighter and more extreme as you reach the end, a bit like the reader's heartbeat. And of course, as with all heartbeat rates, the number one thing to avoid is flat-lining.