Water pressure high when first opening kitchen faucet then reduces dramatically when fully opening faucet

You have a problem with the faucet valve. Those single-handle faucets have a valve that incrementally increases flow whilst also mixing hot and cold, depending on handle position.

There are various types of mechanisms to accomplish this, but a common one is the use of a hollow or solid ball with holes/ports that line up with the incoming water ports to varying degree, and lead to the spout (I mention this because your looks like that type, common manufacturer is Delta). Nevertheless, you need to turn off the water supply, remove the handle, and do some investigating. it could be as simple as a blockage that needs to be cleared, could be a new valve assembly, or could be a new faucet. We will need to see the guts...