Excel 365 is UNUSABLE for professional work — performance has catastrophically degraded

I agree.

I use Excel Online to collaborate with clients on project lists, and the introduction of Copilot has been actively detrimental. It has noticeably increased document load times, and there is still no option to disable it. Combined with recent cosmetic changes, this adds no real value while making the platform slower and more frustrating for both me and my clients.

If Microsoft insists on rolling out new features, they should at least be optional. Instead of prioritising superficial updates and “nice-to-haves”, focus on performance, stability, and functionality that users actually need. Forcing unnecessary features at the expense of usability is not progress. CoPilot is behind the curve compared with other AI agents around in terms of performance and not everyone wants to relinquish their data into the public domain. So, if Microsoft want to push it on customers, invest in it properly - you have the finance and size to make it work.