WebAssembly is the newest web standard. It features a compact bytecode format, making it fast to be loaded and decoded. While WebAssembly is generally expected to be faster than JavaScript, there have been mixed results in proving which code is faster. Unfortunately, little research has been done in understanding WebAssembly’s performance advantage. In this paper, we investigate how browser engines optimize WebAssembly execution comparing to JavaScript. Specifically, we measure their execution time and memory usage with diverse program inputs. Our results show that (1) when the program input size is small, WebAssembly outperforms JavaScript. However, WebAssembly programs become slower for larger inputs; (2) WebAssembly uses significantly more memory than their JavaScript counterparts. We believe our findings can provide insights for WebAssembly virtual machine developers to identify optimization opportunities. We also report the challenges encountered when compiling the benchmarks to WebAssembly and discuss our solutions.