If data is in cache, it will not be read from the database.
If data is not in cache, it will be read from the database.
As data is written in permanent storage, applying the cache system is not related with the enhancement of writing performance
IMDG : In Memory Data Grid.
Read : It is always read from IMDG main memory, therefore it's fast
Write : Even when configured to synchronize data to permanent storage some products support non-synchronized write. In this case you can expect a very high write performance.