PM: So the question is, do you actually have the numbers to work out your climb performance?
Yes, you may have a lot of work to calculate your climb performance and you may be lucky the aircraft decides to follow the number. But, you can not rely on it and that's why I appreciate this thread. Similar situation may happen to everybody and if I remember this, it'll help.
Just yesterday, I had all calculations done well, fed the iPad with all usual numbers and guess what, the aircraft decided to climb only about half calculated performance. It was on flat shoreland, so no problem at all and the flight was never in danger, but if it would have been in mountains, it would have been troublesome. After landing I checked everything I could guess, but found really not a single bit unusual, so lack of performance is a complete mystery to me.