[bzzzt]

Is it macOS or the monitor not being responsive? Also, if it's slow your laptop might be too old to properly support 4K or you might have misconfigured something.

At least Dell supports their hardware: have you tried updating the monitor firmware or submitting a report?

[piyush_soni]

It's the built-in mac display that goes blank along with the external monitor. On my Intel 2019 MBP it was very slow (many seconds), on this new M3 it's much shorter but still more than a decent Windows laptop which is almost instant.