Correct answer - "The GSI is throttling so you need to provision more RCU and WCU to the GSI" :
If you perform heavy write activity on the table, but a global secondary index on that table has
insufficient write capacity, then the write activity on the table will be throttled. To avoid
potential throttling, the provisioned write capacity for a global secondary index should be
equal or greater than the write capacity of the base table since new updates will write to both
the base table and global secondary index.
Incorrect Answers
"The LSI is throttling so you need to provision more RCU and WCU to the LSI" : LSI use the RCU
and WCU of the main table
"Adding both an LSI and a GSI to a table is not recommended by AWS best practices as this is a
known cause for creating throttles" - It is fine to have LSI and GSI together
"Metrics are lagging in your CloudWatch dashboard and you should see the RCU and WCU peaking for
the main table in a few minutes" - This could be a reason, but in this case the GSI is at fault
as the application has been running fine for months.