  To facilitate lattice QCD calculations of nucleon structute, a set of
quasi-parton distributions were recently introduced. These quasi-PDFs were
shown to reduce to standard PDFs when the nucleon is boosted to high energies,
$P_z\rightarrow \infty$. Since taking such limit is not feasible in lattice
simulations, it is essential to provide guidance for what values of $P_z$ the
quasi-PDFs are good approximations of standard PDFs. Within the framework of
the spectator diquark model, we evaluate both the up and down quarks'
quasi-PDFs and standard PDFs for all leading-twist distributions (unpolarized
distribution $f_1$, helicity distribution $g_1$, and transversity distribution
$h_1$). We find that, for intermediate parton momentum fractions $x$,
quasi-PDFs are good approximations to standard PDFs (within $20-30\%$) when
$P_z\gtrsim 1.5-2$ GeV. On the other hand, for large $x\sim 1$ much larger $P_z
> 4$ GeV is necessary to obtain a satisfactory agreement between the two sets.
We further find that the Soffer positivity bound does not hold in general for
quasi-PDFs.
