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A blocking call occupies an executor worker without yielding, so unrelated tasks can stall. Move CPU-heavy or blocking work to a dedicated pool and await its result.
An executor worker can poll many async tasks because each task yields while waiting for nonblocking I/O. A synchronous sleep, filesystem call, expensive compression loop, or blocking foreign-library call can occupy that worker instead. Other ready tasks then wait even though they could run. Putting async on a function signature does not change a blocking call inside it into cooperative work.
Use the runtime's blocking pool or a dedicated worker thread for truly blocking operations, and pass owned input into it.
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