Improvement

Wasmtime v43 Config::new() enables concurrency_support option by default as per the config . This configuration is shipped in wasmtime-v43.0.0-aarch64-macos (tested). In practice, this causes synchronous function calls to have overhead even in Component model binaries that do not use asynchronous features. The culprit seems to be that the FACT module ends up running this code on every synchronous cross-Component function invocation.

After testing a program that repeatedly invokes an empty function over Component boundaries with and without the command line option -W concurrency-support=n, I can see a 50x-100x increase in latency with concurrency-support enabled.

Unfortunately I cannot share code.

Benefit

Disabling the option by default has a significant performance benefits for synchronous Component-model applications that do not need async features. The option should be set only when the used Component needs asynchronous features.

Implementation

Disabling the option in the config by default. Prompting users that want to use async components to enable the feature.