@kop My Cloud has it’s own database outside of Plex and Twonky.
The My Cloud Indexing and Generating previews is only needed if you want to view your content in the My Cloud Web or Mobile App. If you do not need to view your content in Web or Mobile app including Auto Backup of mobile device, then disable Cloud Access.
Plex and Twonky both provide very similar functionality (media serving) but in different ways. However, both each have their own indexes as well. Twonky for general DLNA purposes and Plex for others. Plex is normally used with Plex client apps though. Plex also has DLNA feature (disabled by default) and uses lots of CPU and memory.
But yes, both Plex and Twonky will contribute to disk I/O, CPU and memory usage.
If Plex or Twonky write any data to the shares, then My Cloud Indexing will happen because new content will need to be indexed for Cloud Access.
5.12.108 was just released and includes lot of fixes.