If you go fast, fast, fast, fast you begin to lose impact. It's like someone shouting all the time; after a while you switch off. If you go slow, slow, slow, slow, your readers will begin to drift off.
You need to go forwards in a mixture of fast and slow scenes, though not in such an obvious pattern as fast, slow, fast, slow, fast, slow which will become predictable. And as you get towards the end, the chances are that you'll have more fast scenes than slow ones.
So, what makes a scene fast? Usually lots of action and dialogue, and exciting things happening. A slow scene will more likely include a lot of internal thought and reflection on what's just happened.
If you're unsure, try listing your scenes on index cards. Then lay them out on the floor or a big table along an imaginary central line. Scenes above the line are fast (and the further above the line, the faster they are), scenes below the line are slow (and the further below the line, the slower they are). Ideally, your index cards should zig zag across the floor or table in a varied and unpredictable way.
This is an easy way to check your pace and see if there are any places where nothing much happens for a while (ie several cards together below the line) or if there are clumps of action and excitement (ie several cards together above the line). You can do it for each scene too, and it should show a similarly varied pattern.
And finally, you can check that all your best bits - the ones that you rate highest up the excitement scale - are spread out throughout the novel.
Everything needs to have light and shade and a change of pace to them. Think of a film like Die Hard. Yes, there are bangs and explosions and exciting stuff happening. But there are also sections where Bruce chats to the policeman in a reflective way, the calm before the next storm. Slow, slow, quick, quick, slow or quick, quick, slow, quick, slow, quick - it doesn't matter what the order is so long as it is there.