I tried fruitbats 2.54pm suggestions and got some reasonable results. The only way I have ever found of making any big difference is reinstalling the OS but a lot of time and trouble, it depends on how much it matters to you. I can put up with mine at the moment but I know a reinstall would be better. If you decide to go for the reinstall option it is not that difficult just very time consuming and tedious. Always plenty of free help available here.
iMac Pro review