JRuby inherits the JVM's ability to run threads truly in parallel with shared mutable state, with no Global VM Lock. A JSON-parse demo parsing a 1 MB file repeatedly shows C Ruby threads barely using the extra cores (~1.6× speedup from JSON C code parallelism), C Ruby Ractors reaching ~3.7× on a 4-perf / 6-efficiency-core machine (at the cost of making data ractor-safe and giving up access to mutable main-thread state), and JRuby threads saturating all cores (~6× on the same machine and scaling linearly to 50–100 cores).