The warm up speed sounds great, especially if it's the first start of the day. 70 degrees F. ambient temp, 40 degrees C after 2 minutes, almost sounds "too" good, but if it doesn't overheat, who cares I guess. The oil won't make that much difference, if anything "slipperier" oil will reduce friction in the engine and it'd warm up more slowly if anything.
The upshift delay shouldn't explain not being able to not accelerate more than 5-10 mph as you state. may be heading for trouble after all. i'd leave it at a dealer or trusted repair shop familiar with MB and have them give it a roadtest with the intention of diagnosing the upshift delay. If your "target" speed is 10-20 mph, maybe it's normal. If your target speed is 20 mph or more, then I'd say there is a problem. May just be the throttle valve cable is set too tight to begin with. Have you had the transmission fluid and filter change done recently? Should be done every 30,000 miles.