We investigate the effects of a large-scale interplanetary disturbance, such as a merged interaction region (MIR), on the properties of the heliospheric termination shock and particle population accelerated at the shock. The interaction process between transient structures and the shock is reflected in energetic particle spectra through variability in shock location, its compression ratio, and particle mean free paths that control their acceleration rate. It is shown that particle distributions observed at a specific moment of time exhibit features, such as spectral breaks, from earlier acceleration stages with lowest energy particles produced most recently and the more energetic anomalous cosmic rays (ACRs) accelerated at earlier times. We also show that a MIR collision can produce an extended period of anomalously low radial velocity in the heliosheath if density and velocity variations are correlated within a MIR and a following rarefaction.