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. However, up to Duktape 1.1 there has been very little performance work and Duktape is significantly slower than e.g. Lua right now. Performance work has been scheduled for spring 2015.
See How to optimize performance for discussion of Duktape performance characteristics and hints to optimize code for performance.