Berny,
To help answer your question:
My wife and I get to the South of France, or North of Spain, in two easy days. We tour and get back to the ferry inside a week, no problem. We have developed a routine of driving for an hour and a quarter then taking a break, the first for morning coffee, another drive then lunch, another then afternoon whatever, then a final drive and arrive at destination. Total driving is 5 hours and we are never cramped, stiff or sore. With the aid of modern technology we have our overnight hotel pre-booked and sometimes even a restaurant. GPS gets us to the door. Big change from our early forays years ago when we might find ourselves looking for a campsite in the pitch dark and it takes all the stress out of it. We find that the speed limits (130kph on the motorways) are good enough for very comfortable traveling.
Another angle - I regularly compete in a gliding competition in the middle of Spain. This involves towing a trailer all the way. We get moving from Roscoff at about 11.00am and we are usually in Bayonne - right at the Spanish border - and settled in for an evening meal at about 8.30pm. The speed throughout would be about 60 mph.
Speed is not the way to get there faster. In fact I have to laugh when crotch rockets piss past me because I know I'll be passing them in about 20 minutes up the road! The trick is to keep the wheels turning!
Hope that helps.