The systemcache/local folder is used as a temporary offload of all the elementdata queries before we push them into MySQL (it's faster to push a bulk of data than to open a connection for one query). For elementdata this means the files will contain all changes of saved parameters for every protocol of every active element of that DMA

Having a large folder usually means you're encountering one of the following issues:

1. There was a brief period the files couldn't be put into MySQL (make sure there is no connection drop in MySQL) (usually indicated by files older than 5 min).
2. You have too much activity on the saved parameters (double-check the drivers to see if all the saved parameters should be saved.
3. You might have tables that are too large (each cell is one row in the DB, and thus one line in the file; if you have tables with +100k rows you should consider using a loggertable).

When a DMA starts up it will first check if there are still files present in the system cache folder and offload these first. When there are a lot of files present this will indeed hinder the DMA from starting up (stuck at 99%) until all files are offloaded.

To speed this up you could copy these files from the offload folder so it’s empty again and then put them back once the DMA is started up so it will continue offloading to DB.