Runtime security checks are not necessary for WebAssembly to be safe. You could compile from a memory-safe source language to start with and add support in the

Runtime security checks are not necessary for WebAssembly to be safe. You could compile from a memory-safe source language to start with and add support in the WASM format for proof-carrying code. Then all that's needed in the WASM runtime is a static check of the added proof information, which could be very quick. Runtime checks could still be used to run WASM that doesn't carry proofs of safety, keeping the same performance cost. (Some occasional runtime checks would nonetheless be needed, since safety against e.g. excessive stack use could depend on what the program is doing at runtime; but they would be quite rare and come with very little overhead.)