Stale rate is the total number of shares submitted after the previous block has previously been discovered and pool has moved on to the next block. When everything is working correctly, the stale rate should be a very low number. A number of factors can lead to an increased stale rate:

Network Latency: On a slower network, it will take more time for a miner to be notified of a new block. This leads to the miner continuing to submit results to an old block, after which these are rejected by the pool and viewed as being stale shares.

Network Latency: On a slower network, it will take more time for a miner to be notified of a new block. This leads to the miner continuing to submit results to an old block, after which these are rejected by the pool and viewed as being stale shares.

Slow Mining Devices: A slower mining rig typically needs to complete one task before beginning a new one. While other miners are generating shares for a new block, those with a slower mining setup are likely working on shares for an old block. These, of course, can’t be accepted by the pool and are therefore referred to as being stale.