don't worry about how long your HR takes to ramp up. Only pay attention when it isn't doing what it normally does. there are days my HR responds really quickly because I am still cooked from my last ride and I'm not recovered, so just because your heart rate goes up quick one day might not even indicate a good thing

you get faster by riding faster, that's for sure, but don't think about it as being faster for the entire ride. Be faster for a short stretch. We used to call them "jumps". Its like intervals, but not as strict or structured.

When I was training for time trials, I would try and get faster by riding really fast for a mile or so, not by trying to ride the entire 40k distance faster

And make sure to do some LT time like umd says. This doesn't even have to be very structured. just make sure you are hurting somewhere on the route if you have a hard time with doing formal intervals

in the end, speed hurts, so just how fast do you really want to go?