Accelerator programs provide valuable market feedback and education to participants that may improve startup performance. However, it is unclear whether the average effect of accelerator participation on startup performance post acceleration is positive, and if so, how this effect varies with accelerator program design. We analyze data from 8,580 startups that made it past the initial selection stage at 408 accelerators in 176 countries between 2013 and 2019. We compare accelerated and non-accelerated startups and find a positive average effect of accelerator participation on startup performance post acceleration. Moreover, we find that this effect varies substantially with program design, and depends on venture stage, industry, and founder expertise. Our findings highlight the impact of program design on the benefits that startups derive from accelerator participation.