I think that I and all non-technical folks around me experience issues with applications performance daily. I think that sometimes "making it work" should include some particular performance metrics. If it is not fast enough it doesn't really work. Now "fast enough" is something to be defined and different depending on the application part.
Far too often I see applications that assume low latency and unbreakable Internet connection. They seem to do almost no caching at all. For example thumbnails.
Also many of applications will be almost unusable (or trigger OOM) when you try to work with a big file. Sometimes a big file has merely tens of MB, sometimes problems start with a 3MB file. Those are the issues that occur without thinking about performance from the start - memory is free, you can copy things around, everything will fit in RAM.
One more thing. When your application consists of a client and a server it may turn out that you will put yourself in a corner when not thinking about performance early on. Everything will work without any troubles at first and then it turns out there are some latency issues with more data and you can't easily upgrade the client for example. Or you had an architecture that allows to spin up more servers and handle the load closer to the client, but it can cut your margins.
Far too often I see applications that assume low latency and unbreakable Internet connection. They seem to do almost no caching at all. For example thumbnails.
Also many of applications will be almost unusable (or trigger OOM) when you try to work with a big file. Sometimes a big file has merely tens of MB, sometimes problems start with a 3MB file. Those are the issues that occur without thinking about performance from the start - memory is free, you can copy things around, everything will fit in RAM.
One more thing. When your application consists of a client and a server it may turn out that you will put yourself in a corner when not thinking about performance early on. Everything will work without any troubles at first and then it turns out there are some latency issues with more data and you can't easily upgrade the client for example. Or you had an architecture that allows to spin up more servers and handle the load closer to the client, but it can cut your margins.