Performance tuning is the work of finding where a live system spends its time under real load, in queries, requests, and assets, and removing the cost, so software that works correctly also feels quick to the people using it.
Let's TalkPerceived speed is most of what people mean when they call software good or bad to use.
Fixing the cause is usually cheaper than paying for larger infrastructure to hide it.
Busy periods stop being the times the system is least usable.
Slow software breeds workarounds and shadow spreadsheets, and those cost more than the slowness did.
This is one part of our maintenance and support work. Engagements rarely stay in one box: a platform rollout turns into something custom, and both then need looking after. That is why our software development and implementation practice treats implementation, custom development, and ongoing support as one continuous piece of work rather than three separate vendors.