Internally Temporal relies on durable timer queue abstraction. So all these sleeping workflows are going to have a task scheduled to be delivered at that timestamp. When time comes the task is delivered, the appropriate workflow is updated with “TimerFired” event, a workflow task is put into workflow task queue. Then your workflow worker are going to pick it up, recover workflow to its last state and execute doSth() operation.
The only problem with scheduling a very large number of workflows to wake up simultaneously is that other namespaces hosted by the same cluster might experience a slowdown in task processing while these millions of timers are executed. In this case, we recommend to jit them to some reasonable period of time (let’s say to fire during a 15-minute window). If your cluster is not multi-tenant then I don’t see any issues.