Slow downshifts?

2011 E550 4Matic 35k miles. Runs like a charm. Except, on the highway if I take my foot off the accelerator and I'm cruising (e.g., 50-60mph) and I floor the accelerator, there is a NOTICEABLE delay before it drops down to the next gear and gets me going. If feels like forever, especially when the car in the next lane is approaching. Has anyone else experienced this?