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WebAssembly (Wasm) was created initially as a browser-based execution environment to achieve performance close to native for web applications. But with the maturation of the WebAssembly System Interface (WASI), Component Model and production-grade standalone runtimes, Wasm has become a universal cloud runtime, capable of running the same compiled binary on cloud servers, edge nodes, content delivery networks (CDNs), IoT devices and even within browsers. This paper presents a comprehensive survey and performance analysis of WebAssembly as a cloud runtime. We synthesise results from academic literature, industry surveys, and production deployments, comparing Wasm against traditional container technologies (Docker, containerd) and other lightweight execution environments (unikernels, microVMs) along key metrics: cold-start latency, binary size, memory footprint, execution throughput, and security. Our analysis shows that WebAssembly has 50-200× faster cold starts (0.05-5 ms vs. 100-500 ms), 50-100× smaller binary sizes (100 KB-5 MB vs. 100-500 MB), and 10-50× lower memory consumption (1-10 MB vs. 128-256 MB per instance) than containers. The capability-based security model of WASI offers better default isolation than container namespaces. But Wasm is 15–36% slower for compute-intensive workloads and has up to 2x overhead for I/O and serialisation. In conclusion, we determine WebAssembly is most suitable as a complementary runtime to containers for stateless, event-driven, high-density, and edge-native workloads, whereas containers are more suitable for long-running stateful services and I/O-heavy workloads.