I always rent a car while visiting Las Vegas. The whole process is usually under an hour from landing at the airport, retrieving luggage, catching the shuttle, renting the car and driving out of the rental complex. Here's a scan of my receipts from my last trip. I've scanned only the dates & time. On the top is the itinerary of my flight and the bottom line is the date/time I left the rental complex. I landed at 6:35 am and drove out at 7:30 am:

On this last trip I didn't have "express" service, I had to wait in line to rent the car. Otherwise it probably would have been 10 - 15 minutes faster.

Prepare yourself with maps or a GPS unit and driving is pretty straightforward in Las Vegas. The only unexpected thing is the traffic can be very heavy at times.

I'm not a convertible person but I would think with the top down it's like driving through an oven.