Antiferroelectric (AFE) ceramics are attractive candidates for high-energy-density storage applications, particularly in pulsed power systems, due to their unique field-induced phase transition. However, optimizing their performance requires a delicate balance between maximizing recoverable energy density ($W_{\rm rec}$) and minimizing hysteresis loss to ensure high efficiency ($\eta$). While experimental studies suggest that microstructural engineering is a viable optimization pathway, the microscopic mechanisms governing the interplay between grain size, elastic constraints, and domain switching remain poorly understood. This work employs a high-order phase-field model within a Fourier spectral framework to investigate the electromechanical switching behavior of polycrystalline PZT-based antiferroelectrics [1, 2]. We systematically vary the microstructure from single crystals to fine-grained aggregates to isolate the effect of grain size. The simulations reveal that grain boundaries act as potent heterogeneous nucleation sites for the field-induced ferroelectric phase, significantly reducing the switching barrier compared to single crystals. Consequently, coarse-grained polycrystals exhibit a marked increase in $W_{\rm rec}$ relative to the single-crystal baseline. However, further grain refinement introduces a critical trade-off. In fine-grained microstructures (e.g., 36 grains), the dense network of grain boundaries and triple junctions creates strong mechanical constraints. We visualize this effect through mapping of stress components ($\sigma_{11}, \sigma_{22}, \sigma_{12}$), which shows intense stress concentrations at boundaries that permeate the grain interiors. This clamping suppresses long-range domain ordering, stabilizing a frustrated, vortex-like nanodomain state. While this state facilitates tunability, it degrades energy efficiency due to increased internal friction. Our results identify an intermediate "medium-grained" regime (approx. 12 grains in our domain) that optimizes this trade-off, achieving a maximum efficiency of 77\% with a high $W_{\rm rec}$ of 6.0 J/cm$^3$, aligning well with high-performance experimental ceramics.