Sync requires 1.5 - 2 Kb (1.5Kb on average) of memory for every file and folder it manages depending on the actual path length. The bigger folder tree you have, the more memory will be required to build database. Sync always keeps the whole tree of files in memory for fast operations. Also, all file operations are kept it database. Even if the file or folder is already deleted, they are still stored in DB, marked as "Deleted".
That is why the DB is constantly growing as you use share and files activity is going on there.
If there is not enough RAM on your device to keep Sync running, OS will unload some of Sync's data to a swap file, which will cause the whole device and Sync to run extremely slowly.
To avoid this situation, either extend the physical memory of your device, or free some RAM - remove the biggest folders from Sync from all peers, and share the folder again. As you remove, the database will be also deleted. As you add it back, new database will be created (Sync will have to index the files) and it will only keep the current set of files.