This error refers to one of the agent's activities as described here and means that it requires some more time to do the operation. The error is intermittent and does not necessarily mean something critical. Agent can restore itself from this state. Depending on the operation mentioned in the error message, admin can identify the narrow place in job execution. This may be clogged disk, script execution, availability of files themselves.
In Resilio Connect version 2.12 the error format was updated and the message changed to "Agent underperforms on a certain operation". Also, three new operation are reported, indicating that the Agent is busy collecting disk information: Getting disks stats, Getting cpu load, Getting disk info.
If the error does not go away within reasonable time, it's better to contact support with some details about the job.
Error code SE_STUCK_JOBS_ACTIVITY_ERROR.