My cheap air compressor was taking a long time to pump up. So I investigated and found some strange things that impede air flow. This Instructable will show you how to make your air compressor pump up faster. This particular air compressor was purchased from harbor freight.

So the original air filter has an indirect air path to the foam filter. This is to make it quieter. But its a loud air compressor. If you want more flow with out voiding your warranty just flip this cover over and air can come in from any direction.

Also in the third picture the highlighted area shows where the plastic was covering the air inlet. This was from super high quality Chinese manufacturing.