Beware of HR Lag!

It is said in the Cyclist Training Bible that, the heart rate monitor is no good for short and intense effort, because there is a lag between the time when you start the interval and the time when you actually see your heart rate in the training zone, which may be 1 minute or more. So you could have a low heart rate at the end of that killer interval; Or worse, never achieve the right training zone!
In my case, when I started the M6 workout of 25 minutes at Zone 5a, it took the heart rate about 2 minutes to climb to that zone, as the graph below indicated.


And when doing the A2 workout of 3 minutes at Zone 5b with 3 minutes recovery in between, the heart rate monitor is totally useless. Again, it took the heart rate about 2 minutes to climb up, just about when I ended that interval. And the heart rate wasn’t even in Zone 5b! However, my power output did show that I was training in the right zone. (See graph below)

So, the power monitor is indeed a better tool for monitoring intensity. If I had the money, I’d invest in a $ 1500 Powertap SL wireless or SRM, instead of a better bike. (That said, I already have a pretty decent bike, thanks to my secret benefactor)
P.S. Graph was made with Polar Precision Performance software that came with the HRM. Elite Primo trainer did not offer option to download the power data, but it should be parallel to the speed data on the Polar HRM.