I wore a HR monitor at IMFL and stayed in Zone 1 or 2 during the entire bike leg. My heart rate never ventured above 141 bpm, and at times, my pace felt frustratingly slow. Fortunately, I followed it with a marathon in about what I expected. Finished in 12:06.
With that said, I am definitely not married to my HRM. I only use it for long training sessions and in races 1/2 Iron and bigger.