Is the point of your back end to transform the data?
Transforming data is full blown feature in itself. In fact it could be argued that all any software does is transform data. I presume what we're talking about is transforming what the front-end gives us to what the back-end needs.
You set a performance point saying that the front-end can handle an array of size 100. This is good to know but we're still lacking enough information to make the call. We need another performance point that says the back-end can handle an array of size x (with y simultaneous users).
With that information we could make decisions that would increase the overall capacity of the system. Without it we're just holding to our dogmas and hoping.
There are good dogmas of course that give you good goals to steer towards but since your question indicates performance is your primary concern I advise you to stop thinking about what and focus on how much.
With that answered look at what pursuit of performance has cost your design. We have these dogmas for a reason. Ewan has a good reason to put the workload on the user. When y is large it's a really good reason. But costs come in many forms. Loss of readability and flexibility cost developer time which can easily trump data-center costs. Loss of security can cost you the company.
Every piece of successful code ever written is a compromise. I've never seen anything live up to ideals. I've seen nightmares that desperately need ideals. Don't use performance as an excuse to hide from these ideals.