OpenGL on macOS is no longer actively maintained and its version is quite outdated. I believe switching to Metal could provide better compatibility and significantly improved performance, especially on Apple Silicon (M-series) devices. This could greatly enhance canvas rendering performance and overall responsiveness on modern Macs.
Qt doesn’t support using Metal directly, which makes this request basically impossible in the first place. It’s also questionable whether there’s a point in spending a lot of time and money for to learn, implement and maintain a rendering backend in the unfounded hopes that it will make rendering a 2D textured quad notably faster.
What would be feasible is to use ANGLE, which provides an OpenGL interface on top of Metal. It’s also what Krita uses on Windows to do the same to DirectX. Whether that’s faster or better than just using straight OpenGL is something that can only be measured and might be device-dependent. I don’t think anyone has tried.
Newer versions of Qt also have their own “RHI” rendering abstraction that translates to various backends, Metal being one of them. Implementing a renderer on top of that would at least be possible, but that too requires time to learn, implement and maintain. It’s not currently planned for anyone to work on it.