This is because some of your traffic is going out over the DSL line which is much slower. The default algorithm uses the lowest latency connection and your DSL could be lower in some cases. I recommend to change your default outbound rule to “Priority” and put your Cable connection as priority 1. This way traffic will route across the faster cable connection unless it goes down, then it will use the DSL connection.
Otherwise, what is your goal in using the Balance router?