Believe it or not, connection comes into play here, and is a leading factor. If your connection isn't the greatest, or people have better connection than you do, they're going to get things faster in chests. Has it ever happened when you open a chest, get a sword, a golden apple, and maybe a helmet, but when you check again in your inventory it isn't there? Well, that's because someone got the item(s) faster than you, or just has better connection.
For example, you run to corn, click a chest, but it may take 1/2 second - 3 seconds to open, and if a person gets that chest but they have better connection so they are able to open the chest and shift click faster. However, if connection is not an issue, it may be just the factor that you're not implementing all steps into one while looting a chest. 1. Open the chest (Start right clicking at a good speed before you even reach the chest) 2. Right after you do that, hold shift and click the items out of the chest (You will start clicking and getting the items as soon as the chest opens, for example if you open the chest and your mouse cursor is already on an item you can just click)
You also probably do not have the most convenient setup for doing this. Personally, I find that tab for running and sneak for shift works the best as I can easily loot chests. You're going to just have to practice, maybe change how you loot chests, and the speed at which you do it. I also recommend having your mouse DPI (dots per inch) and sensitivity set to something that's comfortable for you. I have the SteelSeries Sensei Rival, and my sensitivity settings are DPI: 800, Windows Sens.: 4, and in-game minecraft sensitivity which is 121. So in the end you're just really going to have to experiment with settings and find what's best for you. If your connection isn't the best, play on a server that's hosted closer to you.