depends on your target device…sounds like a buffer is filling (giving the appearance of a higher download speed than the target disk can support) and a consequent slowdown when full.
happens to me especially with usb flash drives or some 2.5 laptop disks.
even the lowest end NAS can “store” 4k…or even 8k for that matter.
if you want to run plex or something to playback in real-time, transcode etc. then you might need some horsepower…