Previous research has shown differences in various soy-based proteins based on in-vitro protein hydrolysis (Bible et al., 2023). But, the impact of an ingredient’s hydrolysis rate on broiler performance has yet to be established. The effect of two soy-based protein sources characterized by different rates of protein hydrolysis (slow vs fast) in starter diets on broiler performance was evaluated. 1,560 Ross 308 one-day old male broilers with initial weight of 39.5 ± 1.9 g were randomly allocated to one of two dietary treatments (30 pens/treatment; 26 birds/pen). Treatments were: corn-wheat-based starter diets containing 28.95% soybean meal (SBM) and 4.68% of a soy protein concentrate (SPC; slow protein) or 28.83% SBM and 5.00% of an enzyme-treated soy protein (ESPI; fast protein). All birds received the same corn-wheat-SBM grower and finisher diets. Diets were formulated to meet breed nutrient recommendations. The starter diets were formulated to be isocaloric and isonitrogenous with the inclusion of SPC and ESPI balanced on protein contribution. Birds were weighed on d 10, 28 and 42 and overall unadjusted (FCR) and mortality adjust- ed feed conversion ratios (mFCR) were calculated. Protein hydrolysis rate was determined for the two starter diets in-vitro using pH-stat method as described by Bible et al., 2023. Performance data were analyzed using GLM in the statistical software R (v.4.0.2) with means separated using Tukey’s test. pH-stat data were analyzed using a pairwise t-test. Signifi- cance was declared at P<0.05. The speed of protein hydrolysis (k-value) was greater with ESPI in the diet compared to SPC (111 vs 85 μL/mole∙s; p=0.04). Birds fed ESPI weighed significantly more on d 10 (271 vs 248 g), 28 (1.58 vs 1.48 kg) and 42 (2.99 vs 2.89 kg) compared with the SPC- supplemented birds (p<0.05). Overall FCR and mFCR did not differ significantly (p>0.05).