One thing I find that works well is having six solid fuel boosters around a central core, attached using the decouplers (not the round decouplers, but the ones that sit on the side of the fuselage and hold things). You split the six boosters into three stages, each with two boosters (the two must be on opposite sides). You fire two boosters at a time until they run out, use the decouplers to get rid of them and then fire the next two. I find this gets you much higher than just firing all six together - I assume because each stage has less weight than the previous one, as you're getting rod of the boosters after they go empty.
I haven't noticed any slowdown and I play at the same resolution you do, but perhaps you're building more complicated rockets than I am. Or you're playing the demo and they've optimised things since then? Not sure.
