Throughput is bounded per partition, not only per table. A single partition supports 3,000 read capacity units and 1,000 write capacity units per second. Adaptive capacity can shift unused table capacity toward a hot partition up to that ceiling, but it cannot exceed it, so a table provisioned at 40,000 WCU still throttles at 1,000 WCU when every write shares one partition key.
This is why key design dominates DynamoDB performance. Choose partition keys with high cardinality and even access, and when a naturally hot key is unavoidable, write-shard by appending a bounded random or hashed suffix and fan out reads across the shards.
Two related limits shape the same decision: an item is capped at 400 KB including attribute names, and an item collection (all items sharing a partition key) is capped at 10 GB only when the table has a local secondary index. Without LSIs there is no per-collection size limit, but the per-partition throughput ceiling still applies.