When I opened up the laptop clean without all my ported data Alchemy ran fine. Never tried running Renoise on it, though.
I think you experienced a “random” kind of change, e.g. Spotlight was not indexing in the background, so more cpu was available, or similar. The current and last macbook (pro) generation has some real design flaws (too thin and poor thermal ventilation). It is said to be not capable of running the cpu at maximum speed for more than 1 minute, then the thermal control in the device will already downclock the cpu quite drastically. Since the Renoise GUI is basically drawn by CPU (I proofed this by doing some tests, although I found CoreGraphics and Quartz? references in the code), the lower the CPU speed, the less fast the GUI will be. Single core speed is the key.
I also have this (maybe totally wrong) theory: Apple once did gpu accelerate CoreGraphics or Quartz. But with the introduction of the new Metal framework, they switched CoreGraphics to CPU, since also older Macs will then “feel” older, too. Esp. if you are using a normal CoreGraphics GUI with Retina, and the system doubles the pixels, it seems to be done fully by CPU. Only if the developer now completely switches to Metal 1/2, it will be fixed (OpenGL was discontinued by Apple, too). This is kind of loose/loose-situation for all developers targeting macOS: Supporting another proprietary Apple graphics API has been proven as not a good idea. Apple could decide in 3 years that Metal 2 is outdated. I think in a situation like Taktik is with Renoise, he seemed to write the whole GUI system himself, it must be pretty frustrating. Such amount of extra work, only because Apple does very bad decisions.