Photoshop 2018 running SOOOOO slowly

Hello

Ive read lots of the threads and followed adobes "Optimise Photoshop" but PS 2018 is still running so slowly.

I have increased the memory usage to 85% which is 18853 MB now and turned off the "use graphics processor" and tweaked the cache settings to suit the types of files and edits im doing. But it is still unbelievably slow. I am literally just adding some text to an image and when typing the word i have to wait 3-4 seconds every time i type for the PS to register what im doing and catch up. This is unusable at the moment.

I know i don't have the faster computer in the world but SURELY it should be fast enough to cope with Photoshop and the simple edits im doing. I have the option of 8GB additional RAM upgrade. Do you think that will make a big difference or am i seriously starring at a whole new upgraded computer requirement??

System I'm using;

iMac (Retina 5K, 27-inch, Late 2015)

4 GHz Intel Core i7

24 GB RAM 1867 MHz DDR3

MD Radeon R9 M395 2048 MB

Thanks in advance