Frontend

A comprehensive guide on testing in React from Johannes Kettmann. A single long-read that covers it all.
Did you ever hear of CSS Houdini? Lisi Linhart did not just hear about it, but went above and beyond and wrote a great piece on its performance.
Adobe open-sourced its own design system and state management library for React!

Node.js

Moment.js, Luxon or date-fns? Hopefully, one day there will be a built-in API that if not removes but at least reduces the need for third-party libraries to manipulate date and time. Here is a TC39 proposal for the new Temporal API that is destined to fix JS Date.
Nowadays almost every Node.js developer has a good understanding of the event loop principles. Yet, only some folks understand how node works from the low-level perspective. If you want to learn more about non-blocking I/O, watch this interesting talk by Sam Roberts.

Some of java.util.concurrent.atomic classes expose #lazySet() methods which may be a significant performance win for a single writer scenario in the concurrent mode. If you are not familiar with those methods and want to understand how they differ from ordinary volatile writes take a look at this post and this one as well.
Everyone knows Netty, which is used as a building block in many other frameworks. One of the main reasons behind that popularity is performance. Watch this talk to learn about some optimizations inside Netty, including the "do not try this at home" ones.

Linux kernel scheduler has to make a lot of decisions and it has to make them fast. Here is a detailed explanation of how the scheduler works.
Writing correct concurrent code is hard, no doubts. Writing performant concurrent code is even harder. If you aim for both of these goals, read this awesome blog post that compares the costs of different concurrent primitives.