RStudio 2025.09.1+401 for Mac is using the Electron 37.2.6 framework, which is known to trigger significant performance issues in macOS 26 (Tahoe). Apparently, the issues have been corrected in newer versions of the Electron framework.
https://github.com/electron/electron/issues/48311
https://mjtsai.com/blog/2025/09/30/electron-apps-causing-system-wide-lag-on-tahoe/
I raised a bug on Github for Positron a couple of days ago. The Posit team are aware of the issue and are working to fix it. I don't know what their plan is for RStudio but I would guess its being worked on as well.
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If you want to give it a try, the daily build of Apple Blossom (2025.12.0-daily+274), uses Electron 38.4.0.
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We will be releasing another patch (2025.09.2) of the current RStudio release to update Electron. Should be released "very soon". Tracking issue High GPU/CPU utilization on macOS Tahoe (RStudio Desktop) · Issue #16495 · rstudio/rstudio · GitHub.
You can try daily builds of 2025.09.2, with the updated Electron, at: Cucumberleaf Sunflower
I tried 2025.09.2+418, but it doesn't launch on my Mac (apparently it can't find a library: "Library not loaded: /Library/Frameworks/R.framework/Versions/4.5-x86_64/Resources/lib/libR.dylib").
That being said, I can see it has Electron 37.6.1 inside—which has the fix 
2025.12.0-daily-274 has Electron 38.4.0 indeed and seems fine
That's... concerning! This is likely the version that is going to be released and has already gone through a bunch of testing. I am unable to reproduce that problem (i.e. it starts fine for me on macOS Tahoe).
The error you shared suggests it was trying to use the Intel variant of R; is your Mac Intel-based or Apple Silicon?
My Mac is AppleSilicon. I migrated from an Intel Mac though so it is possible that some Intel remnants propagated from the previous install.
I reinstalled R-4.5.1-arm64.pkg on the Mac and tried the 2025.09.2+418 again and it worked fine then
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