  Gigahertz acoustic streaming microjets, with the capability of achieving
fluid speeds up to meters per second, open new avenues for precision fluid and
particle manipulation at microscales. However, theoretical and numerical
investigations of acoustic streaming at these frequencies remain relatively
scarce due to significant challenges including: (i) The inappropriateness of
classical approaches, rooted in asymptotic development, for addressing
high-speed streaming with flow velocities comparable to the acoustic velocity,
and (ii) the numerical cost of direct numerical simulations generally
considered as prohibitive. In this paper, we investigate high-frequency bulk
acoustic streaming using high-order finite difference direct numerical
simulations. First, we demonstrate that high-speed micrometric jets of several
meters per second can only be obtained at high frequencies, due to diffraction
limits. Second, we establish that the maximum jet streaming speed at a a given
actuation power scales with the frequency to the power of 3/2 in the low
attenuation limit and linearly with the frequency for strongly attenuated
waves. Lastly, our analysis of transient regimes reveals a dramatic reduction
in the time required to reach the maximum velocity as the frequency increases,
following a power-law relationship of -5/2. This phenomenon results in
remarkable accelerations within the Mega-g range at gigahertz frequencies.
