Considering that there are over 2M immigrants per year and the USCIS staff is about 20K, it's actually pretty quick. If all USCIS did was just immigration it would still be ~100 immigrants per year per employee or about 2 workdays per immigrant. Even considering that some of those are dependents on the same case, it's still pretty fast. But USCIS also handles all the non-immigrant stuff: CoS, AoS, asylums, etc so they don't dedicate their whole workforce to the immigration and the actual caseload is higher than this estimate.
