slow photoshop 2017. iMac (Retina 5K, 27-inch, Late 2015)

I bought the imac. I bought a subscription from adobe. All license. Operating system updated to version 10.12.3 (16D32).

Configuring photoshop compositing acceleration at a basic level. RAM uses 70-80%. Total memory 8 GB in the mac. Feel sufficient deceleration when moving the canvas with the hand tool. Clamp the blank and all are slowly moving. But the zoom works without any problems. When a lot of text is isolated, too, begins to slow down. I use photoshop for web design. Files no more than 40 megabytes at a time. The 3d mode is disabled.

What could be the problem?

3,2 GHz Intel Core i5

8 ГБ 1867 MHz DDR3

AMD Radeon R9 M390 2048 МБ