I found the source of the problem.

Qt5 is likely using libinput by default on Wayland (I have not confirmed this, but it seems likely). The source for qlibinputkeyboard.cpp contains hard-coded repeat settings:

const int REPEAT_DELAY = 500;
const int REPEAT_RATE = 100;

// ...

m_repeatTimer.setInterval(REPEAT_DELAY);
m_repeatTimer.start();

Yup, Qt5 with libinput implements its own repeat using its own hard-coded settings.

So, it seems that, to avoid this, it is necessary to either:

1. Configure Qt5 to use something other than libinput (not sure how to do this); or
2. Fix qlibinputkeyboard.cpp.

Update 2021-01-23: For some reason it works now. The bug is not marked as fixed and the code hasn't changed, so I don't know what the difference is. But the key repeat is fast now.

I found the source of the problem.

Qt5 is likely using libinput by default on Wayland (I have not confirmed this, but it seems likely). The source for qlibinputkeyboard.cpp contains hard-coded repeat settings:

const int REPEAT_DELAY = 500;
const int REPEAT_RATE = 100;

// ...

m_repeatTimer.setInterval(REPEAT_DELAY);
m_repeatTimer.start();

Yup, Qt5 with libinput implements its own repeat using its own hard-coded settings.

So, it seems that, to avoid this, it is necessary to either:

1. Configure Qt5 to use something other than libinput (not sure how to do this); or
2. Fix qlibinputkeyboard.cpp.