“During these 10 seconds of processing, the extension host process burns at 100% CPU and all features that are powered by extensions, such as auto-completion or diagnostics, stop functioning.”
A community extension with six million installs took ten seconds to recolor a 42,000 line file after one keystroke. The extension API gave it no incremental updates and no token data, so it could not do better. Microsoft fixed it by building the feature into the editor. Extensions only get to be as fast as the platform allows.