Mac mini 2 slower than my old 2010 iMac, by miles

Connect the display with a certified high speed HDMI cable and if you need the hub capabilities, get a USB-C/USB-A cable and connect it to the display and be sure to find the setting to set the USB-C port on the display to data only.

In general, the USB-C connection is primarily meant to connect to laptops and when connected to a desktop computer there usually ends up being confusion between the two.

Also as an aside, not all USB-C cables are created equal including so called "top of the line" types especially when required to carry both video and data.

I saw another post here with a very similar complaint to yours in the last month (±) where the the USB cable was not fast enough for the port and monitor being used.

Per the Dell spec sheet, the monitor came with three cables:

Is the C-C the one you are using?

Open Activity Monitor, Show All rocesses, sort on CPU%... is photoanalysisd taking scads of CPU?

There is obviously something seriously wrong with either your M2 mini or the way you are using it.

Without knowing exactly what you are doing it's impossible to hazard a guess.

For a start, download the free Etrecheck, run it and post the full report here.

https://etrecheck.com/en/index.html