same here.
it do no change if you use direct HDMI output from NVR or if you use SmartPSS or Web. This is a Firmware/Software Problem on NVR/IPC. (i would like to have the Firmware open source to fix it.. or see the bugs)
The problem is:
Event: VideoMotion is trigged every time in all cases (you need detection area), this cause a record - after this, SMD will trigger and than IVS rules are triggered and also start "Snapshot" ... the problem: IVS events and Motion/SMD are recorded in seperate files.
usualy it works fine if you have one IVS not overlapping with a other that triggers in a short time distance. otherwise this would cause problem in encoding split up the data streams to sepearte files and cause frames drop. (this is my thereoie ... i can not see sourcecode)
also its recomend to reboot every week the ipc one time. i am not sure why but if you run them too long it also cause lag and short stucks.
the problem is not the record speed of your drive, the nvr has cache/ram and hdd also has. and i use very fast SSD... so its not the drive it's software.
the problem also goes on lower resolution, i do not use 4k (8MP) records, i lowered them to 5MP - thats good enough.