does read(write) capacity unit define minimum execution time for read(write) operation

Question

From aws docs:

One read capacity unit represents one strongly consistent read per second, or two eventually consistent reads per second, for items up to 4 KB in size. If you need to read an item that is larger than 4 KB, DynamoDB will need to consume additional read capacity units.

Confused by bold part, does this explicitly means that reading something over 4KB is not possible if you have just 1 read capacity unit(probably not) or they are suggesting it will be terrible slow(probably)?

For example, having 1 read capacity unit defined on a table i need to read(strongly consistent read) 50KB item, does that mean DynamoDB will need 50/4 = 12.5 => so more than 12 seconds for single read operation?

Answer 1

Basically yes, however DynamoDB supports bursting. It will 'save' 300 seconds of reserved capacity in a pool. If you have 1 read capacity reserved and have something of 9 KB (needs 3 read capacity), then you can still use this quickly as you have 300 read capacity of burst capacity available. You can do this 100 times until the burst capacity is depleted and then you need to wait a while until the burst capacity pool is filled again.

See also the docs on burst capacity: http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GuidelinesForTables.html#GuidelinesForTables.Bursting