I also worry about thunderstorms but I've only been caught a couple of times. I worry more about the lack of visibilty of the traffic around me than anythig else.
You can't go by the weather forecast. In the last 4 weeks they've been guessing. They predict showers everday because it seems train once a day most days but they've been wrong more than they've been correct.
My average speed depends on what type of ride I'm doing. If I go easy on the way to work the 30kms never takes much longer than an hour (ave 28km/h which is the same for a long slow ride). But my best time trial time is well under 50 minutes giving me an average in the 37km/h region. That's including all the traffic lights.
I leave home around 6am so when I get to Jhb city centre it's not too busy at 7am. leave the office at 16h30 (or just before) and usually ride much harder on the way home. In fact I'm going to start getting ready to go in 10 minutes or so. 😉
Roadster, I see you're also having some hassle with the posting process ;D
Don't worry too much about traffic. Ride early before it's too busy and the traffic manners isn't too bad if you ride in a predictable way and use hand signals. They seem to be more or afraid of coming close to me in town.