Topic says it all. Activity Monitor shows literally dozens of “setstoreupdateservice” processes. What are they? A little research has implicated Microsoft.
It may be getting called by something that Microsoft is doing, but setstoreupdateservice appears to be a component of Apple’s private CascadeSets framework. Since it is a private framework, Apple doesn’t document it publicly.
ChatGPT suggested some checks, and pointed to Alfred and PopClip, both of which I use. But restarting without either of those running doesn’t solve the issue (although perhaps fewer of those processes were spawned). ChatGPT also suggested this has been reported as a problem with macOS 27.
CascadeSets in turn is connected with indexing for Siri suggestions and Spotlight.
This article goes deep on how GhostTTY was affected by a similar sounding problem, when running apps that change the title bar of a terminal frequently.