XOSS Cadence, Speed and HR Issues

Hello,
I’m currently use a simple roller and three BT sensors from XOSS (cadence, speed and HR)… My bike is a MTB with 29" wheels… I’m 176 cm and 81kg… I started the program so not able to achieve most than 12 KPH, but in my cycle computer at the same conditions, I’m almost achiving 35 KPH…
Is very frustrating use all that my legs can offer and appears walking guys passing…
Some idea?