Improving rice threshing unit performance and structure significantly enhances the thresher’s overall efficiency by raising its productivity while lowering costs and power consumption. This study evaluated and optimized longitudinal axial flow threshing device performance using two threshers’ structures, namely conical thresher and drum thresher, under different rotating speeds of 1100, 1300, and 1500 rpm and feeding rates of 0.8, 1.1, and 1.4 kg/s. The studied parameters were evaluated regarding thresher throughput, efficiency, seed damage, and specific energy, and it was figured that the experimental parameters greatly affected the whole performance of the threshing unit. The drum thresher increased the threshing throughput from 2304 to 2448 kg/h and maximized the efficiency from 98.6 to 99.07% at 1500 rpm rotation speed and 1.8 kg/s feeding rate. Moreover, it reduced the specific energy from 3.37 to 3.15 kW.h/ton for the experimental variables 1100 rpm speed and 1.4 kg/s feeding. The lowest damage rate of 0.24% was recorded using the drum thresher at a feeding rate of 0.8 kg/s and 1100 rotating speed. These results highlight the distinct effect of the new thresher structure and operating parameters on the rice threshing device’s overall effectiveness for medium-sized combine harvesters.