Hi and welcome to the world of Camunda,
Generally, there is usually a clear notion of a process instance. Separating these instances clearly is a good, i.e., having one instance per token. This also allows cleaning up historic data, which can lead to performance improvements eventually. Or put the other way around: If the database grows too big, the performance goes down.
Asnyc is used to manage transaction boundaries. By default, Camunda tries to make transactions as large as possible. If no error occurs, this can also have a positive impact on the performance. In a case of failure, you risk that more tasks need to be repeated. You need to find the right tradeoff.
The documentation includes best practices on setting transaction boundaries and performance tuning: