Duktape is an interpreted engine with currently no JIT support. It uses reference counting which makes memory usage tight at the cost of some execution performance. Overall Duktape performance should be similar to other interpreted languages.
See How to optimize performance for discussion of Duktape performance characteristics and hints to optimize code for performance.
Profile guided optimization (PGO) is strongly recommended if performance is a major concern. For example, GCC -O2 with PGO can be around 20% faster than GCC -O2 without PGO.