(1) Open webpage with WebAssembly in Webworker which tries to store and load webassembly in indexeddb after feature testing (2) Safari 11 on ios and Mac OS does not support serialization of machine code to indexeddb or load from that indexeddb in WebWorker Result: Startup on second launch on Safari is much slower as compared to Firefox (or chrome canary). On first launch, Safari is relatively faster but owing to indexeddb support, Firefox (and other browsers) look much faster on second launch Expected: Safari should be fastest browser in second launch of webpage.