As an example of the Shimano pecking-order: I had a Tiagra rear derailleur on my new Trek. Came as stock with the bike. I'd push the shifter to change gears, and would sometimes have to hold my thumb while it rattled into the new gear I wanted. This was acceptable. I swapped the Tiagra for an Ultegra - out of pocket - and now it shifts cleanly before I get my thumb off the shifter.