Arduino for STM32

You have to install the serial driver from ST: https://www.st.com/en/development-tools/stsw-stm32102.html
This will allow you to use the USB as serial port.
To program the board you need either an STLink probe or you have to install a bootloader (for this you need anyway an STLink first...).

Please ...

Please post an example sketch which shows the problem.
In general, there can be only 16 bits of any port associated with an interrupt.
For example: if you have an interrupt activated on PA8, this will share the same interrupt on PB8 or PC8 or PD8.

For your information,
I also needed in one project a button library and decided to use the button library you mentioned because it has callbacks for each event.
But it did not worked as I wanted to, in particular it had large time delay to detect a click event.
So I decided to make some changes to ...

We just need to know which core do you use.
see this for your reference: viewtopic.php?f=2&t=301

If it runs on bluepill correctly, then it must be related to the new board and/or STM derivate MCU, so you should check the supply voltages / pins / core settings...

Thanks! I eliminated the lines for beginTransaction and endTransaction, and I'm getting slightly better results (12.5kHz) but I didn't get your first point; are you suggesting to use something like dest=SPI.transfer(zeros, count) instead of
for (uint8_t ii = 0; ii < count; ii++)
{
dest[ii ...

I assume that you are using the official STM core.
Here are some suggestions:
i) You have to look whether there is another block transfer function to transfer array of bytes instead of doing several single byte transfers.
ii) you can use digitalWriteFast() to set CS.
iii) you can skip SPI ...

Don't you have some SPI pins of the F303 board shared with other on-board chips?
Which SPI interface do you use? CS pin?
Which core?

Insert some debug prints into socketSendUDP function so that you know where exactly is hanging
Also, you can add some debug prints in the yield() function.

Btw, which core do you use?

If you are using the Ethernet library form Arduino, then it should not hang at all because the function socketSendUDP() - which is called in udp.endPacket() - has an integrated timeout condition, see here: https://github.com/arduino-libraries/Ethernet/blob/75a3c37b5e513305b82e926ca6a4f8190f536c9d ...