Couple things I would try if you haven't yet. Do an [[How to reset NVRAM and PRAM on a Mac|NVRAM reset|new_window=true]]. That resets boot information, so if there's some weird boot nonsense stuck somewhere, that should clear it.
It could be you do have a short, or some component is just getting super hot now, so the device is legitimately throttling to protect itself from overheating. But I still would want to try a few things first before jumping to that.[br]
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If not, this sounds similar to what happens when a Mac is missing sensor data. Typically this relates to the battery or the trackpad, but in theory, it's possible a missing display could also do this. I feel like I have seen it before, but my memory isn't 100%. What happens if you run diagnostics? Press and hold D at power on, just like you would for any other start up keyboard combo (target disk, startup disk, etc...).