Newer build of firmware V5.02.006 build 240407 available on US and Global EN product pages for DS-7616NI-M2/16P (not on UK page):
DS-7616NI-M2/16P
Also new Hik-Connect V6.2.51 (for iPhone) available today.
I have loaded both... don't expect to see any changes but I will report if I notice...
That seems to be a mystery... I'm thinking the video is being analyzed each time it's loaded... some brute force algorithm but it gets the job done... eventually.
Yes, the latest V5.02.005 build 240318 fixed my playback on Hik-connect... just checked again now to confirm... both regular playback and event playback are smooth as they could be.
There was a Hik-connect update on iOS also... V6.2.50.20240320.3... maybe that needs to be updated also?
It works, but the functionality has changed... it's a pop-up window that shows a picture and related video event... the event video does not start on its own... I didn't use it much, but it seems to me like the older functionally was better... a full-screen live view of the channel on event...
from the EU portal it looks like you are already running the latest/last firmware V4.1.18 build 200120:
https://www.hikvisioneurope.com/eu/portal/?dir=portal/Technical Materials/02 NVR/00 Product Firmware/02 AcuSense NVR/Acusense I/01. 1st Generation Acusense I/DS-7600NXI-I2 4S