Have you confirmed the chrono is reading right or tried more than 1? sounds silly but I have seen it happen!
How much preload do you have on the exhaust valve spring. 10m rifles have a really fast closing valve as they use about a quarter as much air per shot as a 12ft/lb rifle. Most of the conversions involve using a softer exhaust valve spring or reducing the preload on the standard one to slow the valve down. no amount of reg pressure or hammer strike force will get 12 ft/lb if the valve is closing too fast.
Other things to check would be a breech leak, or collapsed hammer spring. Or maybe even the exhaust valve head has become deformed by the seat causing a little step that will sit inside the seat. I have seen this happen on a few rifles over the years, even a very small deformation will restrict air flow when the valve is partially open.
HFT, for people with only 2 buttucks!
VooDoo TwoTwo The next chapter.