I wrote about ordering a new Orbea Onix SL last week. I received the bike, was fitted and off I rode. With three good rides behind me I must say I had no idea a frame could make that much difference. My "old" bike had almost all Shimano Ultegra components, just as the new one does. No increase in speed. It is the handling. No speed wobble on descents. The corners are so much easier.
I guess I knew with a price difference you were getting something, but I had no idea that a person with as little riding experience as I could possibly detect such a change.
For those that may want to know, the old bike is a Tirreno Razza 2000, which is a Performance Bike house brand. It is not a "bad" bike, just no comparison to the Orbea.