  Gradual solar energetic particle (SEP) events, usually attributed to shock
waves driven by coronal mass ejections (CMEs), show a wide variety of temporal
behaviors. For example, TO, the >10 MeV proton onset time with respect to the
launch of the CME, has a distribution of at least an order of magnitude, even
when the source region is not far from the so-called well-connected longitudes.
It is important to understand what controls TO, especially in the context of
space weather prediction. Here we study two SEP events from the western
hemisphere that are different in TO on the basis of >10 MeV proton data from
the Geostationary Operations Environmental Satellite, despite similar in the
CME speed and longitude of the source regions. We try to find the reasons for
different TO, or proton release times, in how the CME-driven shock develops and
the Alfv\'en Mach number of the shock wave reaches some threshold, by combining
the CME height-time profiles with radio dynamic spectra. We also discuss how
CME-CME interactions and active region properties may affect proton release
times.
