"Be careful with mutexes" is good advice, but I'm surprised it doesn't explicitly call out the various channels that tokio provides as alternatives (detailed here: https://docs.rs/tokio/latest/tokio/sync/index.html). There are a variety of options that fit different use cases, and you don't even need to enable the runtime feature to use them (e.g. if you want to do a single check for completion rather than await). I'd estimate that at least half of the bottlenecks I've seen with mutexes when using tokio could have been avoided by not even using a mutex at all and instead passing the data that's truly needed across different tasks with some type of channel.

The other trick I've used a few times that's a bit hacky but can get the job done is when reading a snapshot of the data under a mutex is enough without needing to prevent other changes; if that's the case, you can just clone the data and drop the mutex to allow other uses move forward at the cost of the data potentially being stale.

All of the significant server applications I have encountered in the industry have suffered from the same problem, which surprised their authors but seemed obvious to me: the application was spending the majority of its CPU time doing meta-work like entering and leaving epoll, stealing work from itself, etc. There are principles for writing Tokio servers and these are good points in the OP but I think they are little-known and too easy to violate.

When you're at a point of tuning Tokio, consider taking a look at ef_vi/DPDK + SPDK

For a true high performance you should use thread busy-spinning, CPU pinning and SPSC/MPSC ring buffers.

One great use of agentic coding is being able to add and very granular tracing instrumentation to help with these sort of optimizations.

When working on 20ms audio frames, Trusting MissedTickBehavior::Delay is not enough by itself. If you miss a tick, frames pile up. If you don't want that, you should drain all accumulated full frames on every single tick. Otherwise, a single missed tick can cause permanent latency.

One thing I appreciate here is treating scheduler fairness as something you spend, not something you get for free

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What the hell is Tokio? Articles mentions it like once I was expecting some programing principles from Japan

Fast Tokioo, drift, drift, drift!