Keyboard lag, lost keystrokes, and high fan speed in Filevault's EFI pre-boot unlock/login screen (MacBook Pro 2017) I have a MacBook Pro (15-inch, 2017) with macOS 10.15.4 and FileVault enabled.

When I boot the machine, the Filevault's pre-boot unlock screen appears as normal, prompting me for my password.

Now, while the system remains on this screen, and all the way until the correct password is recognized, the following symptoms occur:


Built-in fans start spinning at top speed within seconds from boot (which, I assume, indicate high CPU use).
Built-in keyboard very noticeably lags: key inputs are buffered before actually appearing in the password text field and some of them are disregarded altogether.


Now, this presents a very practical problem: sometimes it takes me more than 10 attempts to actually get my password to be recognized.
Even worse, with each mangled password the screen almost seem to work slower and slower, all very reminiscent of high-CPU-load situations.

I usually type my password fairly quickly, but any other password prompt within the OS takes it without a hiccup. And even when I slow down, lost keystrokes are still a problem and are hard to detect in a password field, where actual characters are not shown.

The problem repeats itself between macOS installations, and is definitely not caused by system still booting up in the background: as I said, the apparent high CPU load always continues until the password is recognized and stops immediately after.

What exactly does Filevault do on that screen? Is it some sort of deterrence against hacking attempts? Can this behavior be tweaked?
I would try resetting  NVRAM: https://support.apple.com/en-us/HT204063