There's something wrong with your car. I have a 91 300E 4M and its a reasonably fast car. Not great from a standing start but I'd guess the 0-100 km/hr time is around 10 seconds (anyone know for sure?)- just a bit slower than a regular 300E. You should certainly have no troubles merging into traffic at a freeway on-ramp, or passing uphill, etc - and definitely no need to put it in first to get moving. Anyway, get a second opinion on it... and please update, I'd be curious to know what's going on.
91 300 E 4Matic