Big improvement over K610, still slow by desktop standards, but worth the effort to install
This is the fastest stock card that would work in my HP Zbook 15 G2. The Quadro K610M
that was in my G2 laptop would run DX9 but almost zero performance on DX10 and DX11, and
would not play DX12. The K621M is about 3 times faster than the 610 where they can be compared, though only 25% as fast as a good desktop card. But at least my G2 laptop is now functional.
In order to upgrade the card, I had to remove both keyboard and motherboard, the
card is on the reverse side of the motherboard. So it takes some care to make sure the ribbon cables are solidly reconnected on reassembly. But if you don't mind keeping track of a bunch of screws on disassembly/reassembly, this upgrade is definitely worth it for the $35 cost of my used K621M. This card is only compatible with G2. Later ZBooks can handle much better cards than the K621M.
I've managed to do more with this laptop than I could with any other I've owned, plus the upgraded CPU these guys put in it runs like a charm. I have three different OS's running on a 1tb hdd inside, and they all run flawlessly. Thank you guys so much for this wonderful PC.