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

Hi RIM_LLC,

I want to add my voice here to validate your assessment. You have clearly done an exhaustive job troubleshooting—rolling back builds, Safe Mode, and stripping out add-ins are the definitive steps to prove this is not a local issue.

I also want to acknowledge the suggestions from m_tarler and Detlef_Lewin. Their advice is technically correct for standard corruption or calculation bottlenecks, and your responses have demonstrated that your scenario goes beyond that into a platform-level regression regarding how Excel 365 handles GDI+ objects (shapes/charts) in the new Fluent UI rendering pipeline.

Since waiting for Microsoft to fix this could take months, here are two "escape hatches" that I have seen power users deploy to bypass the current rendering bug without abandoning their workbooks:

 1. The "Hybrid Rendering" Workaround

There is a known, if undocumented, quirk where Excel for the Web uses a different rendering layer than the Desktop App. You can sometimes force the Desktop App to "inherit" the lighter web-based rendering state:

• Upload your workbook to OneDrive or SharePoint
• Open it in Excel for the Web (in your browser)
• Click Edit Workbook > Open in Desktop App

This often forces the desktop instance to initialize using a legacy compatibility view for the UI shell (the same rendering path used for older file formats), temporarily bypassing the DirectX/GDI+ lag that affects shapes, charts, and the ALT key. It is not permanent—the state resets when you close—but it can restore responsiveness for a critical session.

 2. The "Dark Mode" Transparency Test

You mentioned using a dark background, which is essential for long hours. The current Fluent UI has a known issue with how it handles "Acrylic" and "Mica" transparency effects—these shaders can create a memory leak when rendering dark shapes over dark grids, particularly with GPU-accelerated rendering.

Test this:

• Go to File > Options > General
• Temporarily switch the Office Theme to "White" or "Light Gray"
• Keep your cell backgrounds dark (that setting is independent)

The logic: If performance returns, we have isolated the cause to the transparency shader layer interacting with your shapes and charts. This won't solve the root cause, but it may offer a usable workaround until they patch it.

I hope these workarounds give you a path forward.