This is going to become my mantra- sounds like it is lean. If the rpm drops when you open the throttle quickly, it's lean. These bikes need to be set up to runner richer than you think is right to have throttle response. There is no accelerator pump and no fuel injection to shoot fuel in. This means that when you are transitioning from a small throttle opening to a large one, it will lean out very quickly until the slide/needle has a chance to react and allow more fuel. How is this solved? Set it to run richer than it theoretically should so that when you crack the throttle open, the fuel is already there.
This will harm your mileage and you will smell plenty of unburned hydrocarbons, but it will run better. This could be the issue with warm up. What you perceive as a long warm up could be due to lean idle characteristics. What are the idle mixture screws set at?
On my bike, set up like I described, on a cool to cold day, it will start up immediately on full choke (enrichener), the revs start to rise after no more than about a minute, then I go to half choke, and about a minute or so later, it's to no choke. I sometimes take off on half choke, sometimes not. If I do, the first time the rpm exceeds about 4500, there is no need for choke and it runs perfectly. If I wait and take off with no choke, the throttle response is a little flat on the first run up the rpm before I shift to 2nd, but after that, everything is warm and perfect.