When your car doesn’t start, you don’t just blindly change the battery, starter, fuel pump, spark plugs or all of the above. Instead, you go to your mechanic and ask him to check what is wrong (or you check it yourself if you are the mechanic) and then fix whatever is broken.
Yet very often I see DBAs doing exactly the opposite with their MySQL servers. Rather than assessing what is the server so busy with, they keep changing configuration options until the problem “goes away”. Alternatively, they add more RAM, more CPUs or faster disks, depending on which resources seems to be the most busy at a time. Or they switch to a new server altogether.
MySQL (with a help of some tools) has a really convenient way to analyse the workload and see clearly what exactly is MySQL so busy doing. And even how much improvement you can expect by, say, fixing a specific MySQL query.