ok so I debugged this the whole day now and by accident I discovered that it’s perfectly fine when using board → Seeed nrf52 boards but is slow as hell when I use board → Seed nrf52 mbed-enabled boards
…
which I think I have to use because I need to use ArduinoBLE which gives me a compile error on the non-BLE variant:
library ArduinoBLE claims to run on samd, megaavr, mbed, apollo3, mbed_nano, mbed_portenta, mbed_nicla, esp32 architecture(s) and may be incompatible with your current board which runs on nrf52 architecture(s).
'SerialHCI' was not declared in this scope; did you mean 'Serial'