This has been observed by other people, so probably unrelated: Bandwidth utilization comparison thread - #1665 by storaje
Regarding your node problems, the most typical scenario in which a storage node consumes large amount of memory (and coincidently getting its UI slow) is when the drives are not fast enough to cope with traffic. By large I mean more than a gigabyte. This might be the case with SMR drives, using write-inefficient parity schemes or using btrfs as the file system, and can be worked around by moving databases to a separate, faster storage (maybe an SSD, maybe just a separate drive) and reducing the allowed number of concurrent connections.
I don’t know what kind of diagnostics does Synology provide, but on Linux the easiest tool to confirm or deny this scenario is to run iostat -dmx 30 and observe the %util column. If it is consistently above 50%, that would be it.