It's never taken me anywhere near 15 minutes to get from the parking lot to TTC, and the monorail comes so frequently I have a hard time imagining that it takes more than 10 mins to get from TTC to MK, assuming you had to wait for the monorail to show up!
WE have found that staying onsite is generally faster, but not always.
We stayed offsite for part of our trip last fall, and we did lose quite a bit of time, I felt, coming and going. We stayed at the Vistana, which is pretty close to the parks. Still, coming and going was a hassle. It wasn't just the drive time, it was the human factor. With a larger group, staying offsite, the tendency was that we had to wait for the last person to finish getting ready. The rooms there were nice, but something about them sucked everyone in, esp in the morning. On a previous trip, I stayed at the Marriott World Center with just two other people (business trip) and we had a much easier time coming and going. Maybe part of it was that there was no kitchen in the room, and it was a very non-peak time at WDW. So I would say, it depends considerably where offsite you stay.
Still, if you've ever stayed at the MWC, you'll know that getting TO and from your parked car takes at least 10 min, in itself.
Respectfully, getting from your car to the TTC depends where in the row you park. Yes, it did take us about 10 minutes when we were farther up the rows, maybe more when we had to wait for the tram to fill up.
During peak times in the morning, I have been there when there was a wait to get on the monorail and the ferry.
Getting into Epcot, MGM, and AK once parked, was considerably faster than getting to MK.
We also found that it was much faster to take Disney transport back to a Disney room at the end of EMH night than it was to go back and get the car and drive back to the room. The buses have access to several access roads around WDW that cars do not, for one thing.
Taking Disney transportation can vary widely, too. Getting to MK on a bus that has to loop around one of the moderate resorts, combined with a mid-day wait can take a while. Taking a monorail from GF to MK is almost always pretty fast, unless there's a problem withthe monorail (seen that more than once).
Ganerally though on our last trip (and others), we found that once we switched over to the Disney resort getting to the parks was much faster, due in large part tothe human factor. Instead of waitng for the last person, who also wants to stop off at the drug store....the first group can leave as soon as they are ready...and curiously, the stragglers manage to catch up pretty quickly.