We study the quantum thermalization and information scrambling dynamics of an experimentally realizable quantum spin model with homogeneous XX-type all-to-all interactions and random local potentials. We identify the thermalization-localization transition by changing the disorder strength, under a proper relative all-to-all interaction strength. The operator scrambling has no light-cone behavior and grows almost equally fast in both phases. In the thermal phase, we find that the scrambling dynamics exhibits fast scrambling without appealing to the semi-classical limit. The fast scrambling dynamics always exists at a fixed bare interaction strength regardless of the relative strength in the Hamiltonian. The model also shows faster or slower scrambling dynamics related to the bare interaction strength. We show that the seeming violation of the fast scrambling conjecture arises from the absence of a natural timescale in the infinite temperature ensemble. We suggest that one needs a proper timescale and the corresponding dimensionless time for fast scrambling dynamics. After introducing such a timescale, we give a general phase diagram of the fast scrambling dynamics in the thermal phase. We also briefly discuss the experimental realization of the model using superconducting qubit quantum simulators.
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