On every balancer round, the balancer iterates over all the sharded collections stopping when all shards are already involved in one migration (SERVER-70973).
However, the balancer can still exceed the 1 sec defined interval time between rounds (kBalancerMigrationsThrottling) in the following scenarios:

• The number of sharded collections is very high and it's not possible to involve all shards in a migration.
• The number of sharded collections is very high and just a few of them are imbalanced.