I've written several scenes and reread them silently. Most times they sound rushed, unnatural and uncomfortable. This especially happens with action or other fast-paced scenes. How do I avoid this happening so that the scene develops naturally and doesn't feel like everything is happening too fast? Are there any tips?
In fast paced action scenes being rushed may not be a bad thing as long as this makes sense in terms of the overall rhythm of the story. Indeed as a reader I would say that action scenes are more often too slow that too fast.
Having said that if you do feel a scene is too sparse by all means trust your instincts and play with it and try to work out what essential information is missing but then again it is not necessary to describe every detail exactly as you imagine it.
There can often be a temptation with action to think of it in terms of cinema and want to over-describe it in visual terms by transcribing your exact vision onto the page. One way around this is to make sure that you have already set up the general atmosphere and tone and any details relevant to the mechanics of what is happening in advance then you can rattle through the actual action at an appropriate pace without having to either slow it down for exposition or miss out information which is crucial to understanding what is going on.
It may help to think in terms of the perspective of the characters involved. Ask yourself how would they perceive the situation ? What details would stand out to them and what would they miss or ignore ? If a character is exploring a location for the first time under no particular pressure they may take in a lot of detail but if they are fighting for their life their impressions might be a bit more to the point.
There is certainly a literary equivalent of 'bullet time' where at the crucial moment time slows down but this is probably best saved for really pivotal instants.
Also think about a logical sequence of events in terms of cause and effect, while you might have a series of events in your mind which need to happen to advance the story think about what actually causes them to happen and whether a particular reaction or action is appropriate to the characters as you have established them.