@kirby @why
I have an old HP ProBook and I'm using Micron SATA SSD in it — m510dc. It's as fast as I expect it to be, really great!
This laptop also has an m.2 slot or NGFF as they called it at the time. But it was supposed to be used for a WWAN card, such as 4G modem. There are SSDs that are known to work with this laptop, but according to the reviews they all run really hot — I probably won't take such a risk.
Using m.2 SSD with old laptop is a gamble.