Why issues are missed

One of the main reasons that performance issues are not noticed in development is that some problems are not perceivable on a development system. Issues may not occur until latency increases. This may be because a large amount of data was loaded into the system and retrieving a specific record takes longer. This may also be because each piece of the system is deployed to a separate server, increasing network latency. When the number of users accessing a resource increases, then the latency will also increase.

For example, we can quickly insert a row into an empty database or retrieve a record from a small table, especially when the database is running on the same physical machine as the web server. When a web server is on one virtual machine and the big database server is on another, then the time taken for this operation can increase dramatically.

This will not be a problem for one single database operation, which appears just as quick to a user in both cases. However, if the software is poorly written and performs hundreds or even thousands of database operations per request, then this quickly becomes slow.

Scale this up to all the users that a web server deals with (and all the web servers) and this can be a real problem. A developer may not notice that this problem exists if they're not looking for it as the software performs well on their workstation. Tools can help in identifying these problems before the software is released.