The top quality I look for in a programming monitor is contrast ratio. For me a good contrast ratio has a huge effect on text readability and eye strain.
Panel type matters for contrast ratio. There are really three types in common use: IPS, VA, and TN. I've had great luck with VA panels which are not as well liked by gamers due to somewhat slower refresh but have great contrast ratios. I don't personally notice good VA panels looking "slow." High-quality IPS panels are good too of course. TN panels are very fast and usually are the cheapest but they also tend to have the worst contrast ratio. I'd avoid them for programming.
Beware of over-exaggerated contrast ratio numbers. Look at reviews and third party measurements if you can find them.
I have a Benq 30" 4K VA panel monitor right now for my primary and it's pretty good. Had a Samsung curved VA panel monitor a while ago that was also great but I don't think they make them anymore and mine broke (due to being dropped, not burning out).