Are these other cyclists more fit than you? If you're both spinning the same RPM, but they're going faster, it's because they're in a higher gear. If you ride with someone that's a real masher, they could spin slower than you, but in a much higher gear, and still go faster.
My advice still stands, but I can illustrate it a bit better for you. Find a nice flat stretch of road, and see how fast you can go with that 34x11 combo. You'll find at some point that your legs are spinning crazy messy circles, and your spin is no longer efficient. When you can come close to that on the 42 ring, it's time to change your gearing.