So basically let me get summarise this:

- you have an employee that works faster than others, in the same pay grade.

- You are annoyed that he or she has empty queue of work

- You expect employees to look for tasks on their own when they are done

so either that employee:

- has no creative control over the work he or she is doing and cannot move forwards on their own.

- is severely underpaid and undervalued

- is under bad management that cannot fill their work queue fast enough

All of that will lead into them doing tasks at the same speed as other ones - you just turned a good, valuable and fast employee into another drone.. or worse - they'll go work for competition, and you'll have to both suffer the loss of a good trained employee and a cost of hiring someone else.

If this is happening, it's 100% due to bad management. It means the employee wasn't given creative freedom and a strategic direction to go in. The very last thing any manager should ever do is punish their best employees (e.g. the person who can finish a whole day's worth of work very quickly).