Are you using OpenGL in your SDL app? Or are you using SDL renderer?
If it’s SDL renderer, then the reason is macOS. The way SDL renderer handles vsync on macOS (using the default Metal backend) is that it just sets the underlying CAMetalLayer’s
displaySyncEnabled property to YES, and lets macOS take it from there. For whatever reason, for the first second or two macOS doesn’t really honor that property, and so applications can see their FPS zoom way up at first and then settle at 60. (noticed it myself and dug down into SDL’s Metal code to see why)
If you’re just using SDL to set up an OpenGL window & context, I remember there being some special cases in macOS OpenGL that SDL had to work around.
For your Cocoa + OpenGL application, it’s not surprising to see an application’s FPS drop for the first few seconds either.
Unless you’re experiencing graphical glitches or weirdness outside of your own app, I doubt any of this is caused by your MBP failing.