I don't see the big jump as of High ISO performance. But the AF power of D610 is disappointing for tracking. For a travel camera, D610 is more than enough. But for moving targets, I normally fall back to the old faithful D700 and it's 8 years old AF beats D610 hands down.
The only time the D610 AF has been a problem for me is in low light. The focus assist lamp helps in that situation. I leave it off and have only turned it on a couple times. I don't have problems tracking moving subject (i.e. birds in flight, kids running around). AF-C with D9 does the trick. Here are some pictures I took last week.

here we are at a high ISO. Lost some detail but the focus was spot on. Notice the tip of the wing is out of focus due to the DOF. However, the focus nailed on the body of the bird

Low ISO = good detail. Again the AF worked great
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With respect, shooting slow-flying herons and circling raptors is not particularly challenging, nice as your images are. The flight is slow and 99% predictable. Further, in these examples the birds' line of flight is transverse to the camera/lens, which is again not particularly challenging to follow. Following the play in a football (soccer) game or AFL (USA) game from near range (sidelines) is far more difficult. Or try shooting swallows in flight or even terns near the sea.
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Mike McEnaney. (emem)
www.veritasmea.com
I have shot swallows in flight. They are tough! The problem is if they are close the flight is really unpredictable and I can't track them zoomed in at 600mm with them filling a good portion of the frame. I believe the problem isn't with the AF system unable to get focus but more the speed of the AF motor in the lens itself.
The herons are definitely easier but are challenging in there own right. I would say if I spot one flying I might get 5 seconds to get focus and fire off some shots before they are too far away. I would like to add that if the bird is flying towards me, or away, the composition doesn't do it for me. I like to get the eye in the picture. I actually do have many shots like that where I did get focus but I never converted them from raw.
Really at the end of the day the D700 and other camera bodies probably have more advanced AF systems. In fact the D500 is very tempting for the AF alone. I constantly see the D610 getting slagged for poor AF. I just don't see it. Mine works great. Perhaps ignorance is bliss for me