Why does it take you 15 minutes to park your bicycle? That's definitely an area you can save time on with wheel locks or locks like this one from walmart that attach to your bike and use a key:
https://www.walmart.com/ip/Kryptonit...Cable/17300983
If you can get away with using a backpack instead of a briefcase, then I'd recommend getting the banjo brothers backpack-pannier: (
Convertible Waterproof Pannier/Backpack | Banjo Brothers). All you have to do when you get to work is take it off, flip the flap, pull out the straps, and put it on. Makes life 10 times easier. It's also waterproof. Definitely worth the price.
As for the clothes you wear, depending on how far you commute and the type of bike you're riding, I'd just wear regular clothes. If you have to wear different clothing, then I'd fold the work clothes neatly, put them in a string bag, and bungee them to the top of your rack. It also helps if you wear clothes that you can take off quickly- ie. not skin tight cycle clothing if you can avoid it.
I ride 6 miles to my high school and I always ride in the clothes I'm going to wear that day. I bring a sweat rag that I keep in the side pocket of the banjo brothers backpack next to my waterbottle. When it gets hot (I'm in Dallas-Fort Worth), I wear loose athletic shorts and a tanktop/white undershirt and put my clothes on top of the back rack, and use a camelback knockoff for water (about 2L per one-way trip in 100F heat). Deodorant and bodyspray are the true heros of my daily commute.
If you could manage with a folding bike then that's what I'd do. But it sounds like you like the workout/adrenaline so maybe your current bike is better for that.
Get the banjo brothers backpack though- that will save you soooo much time. Alternately, you could get a front rack and basket so that you just plop your work clothes and backpack in there and go. Just make sure you get a frame-mounted front rack (look for dutch front racks), so that the handling in the front isn't affected too much.