Indeed performance and sizing work-flows is quite nebulous, especially so in file-serving. The reason is that in order to complete an open, read or write operation you need to consider that 33% of the issue could be work-flow or client side, 33% network and 33% server.
The common issue is not describing the performance statement, impact and setting reasoned expectations.
To give you a more specific answer many folks do not factor in the cost of data protection or quota monitoring. Reducing data risk has a trade off in performance.
On workflow the most common area to address is ensuring that the client workflow is scaling out across all the onefs nodes in the cluster in order to leverage all the CPU, net, memory and onefs disk scale-out technology.