Immutant uses Undertow, which will start using Netty under the hoods in 3.0. Perf-wise, both Yada & Pedestal (and many others) add a lot of components to request pipeline by default, so they can be easily an order of magnitude slower than raw Netty. For raw byte-perf, just use the Java servers or lite clojure wrappers, Aleph being one.