What affects the HRM?

I think the above is a pretty good write-up, but just to mention here there are a few factors that delay getting good readings...

• The main one is just the pressure of the watch against your skin - during acceleration that will change, and will change the reading.
  
• However if that reading changes too much the HRM sensor will adjust its exposure to compensate - and if that happens when movement stops it can take a few seconds for exposure to readjust (exposure compensation could be disabled during periods of high movement).
  
• Then there's the median filter. Generally we do just give you whatever reading we can get, but after a lot of bad data as mentioned it'll take at least good 8 readings I think before all the bad ones drop off
  
  

The algorithm can definitely be improved (although I have tried a few different methods already), but a few people have said they'll look into it and sadly there hasn't been anything yet. I did make a repository containing a test harness and data (see http://forum.espruino.com/conversations/373033/#comment16554946) so that could be a good starting point as you can run that just on your PC so can easily make changes and debug it