The low initial ROC is due to the low thrust to weight ratio. The extended low ROC in the video was due to large power reductions required to keep the coolant and oil temps below critical limits.The climb rate was probably kept very shallow for a purpose.
The long takeoff roll could have also been done intentionally the check stability and high speed on the ground just before liftoff.
The lack of acceleration was noted in the previous Raptor thread and is clearly the result of a low power (thrust) to weight ratio. Dozens of fast taxi tests in the vids document this over the last several months with 25-32 second ground runs to reach Vr.