Codex becomes extremely slow and causes severe UI lag when working with many files

No, all of this comes from the Codex application itself. It is most likely related to Electron, so the layout and rendering within Electron may not be well optimized. To be honest, I would suggest switching the Windows version of Codex to WinUI instead. That would significantly improve performance compared to using an HTML-based interface. The lag is particularly noticeable when creating a new thread, where the application may freeze for approximately 3–8 seconds.