On Mixbus, we updated our packages to use a much newer Apple compiler (the one provided with 10.10). That change was made to support the latest Waves plugins which require newer macs, and also to facilitate the signing+notarization of the application for security reasons.
For that build, there is a known problem with older macs (Pre-2018) that don’t fully support Apple’s “Metal” hardware.
If your computer is affected by this “slow drawing” bug (and you didn’t have this problem before now) then you likely need to use the older build which used the Apple 10.6 compiler. A link to that older version is provided in your download email for Mixbus. Note that this build is “unsigned” so you’ll have to right-click to Open it the first time. And it won’t work with Waves v10 plugins.
You mention that this problem also affects Ardour and I don’t think that should be the case. Ardour still uses the 10.6 compiler. IIUC, YMMV, etc
-Ben at Harrison