Memory

If you haven't already done this, I suggest opening the Activity Monitor app in the Applications>Utilities folder. Once the app is open, click on the Memory tab to see how memory is being allocated. Whichever app is at the top of the list is using the most memory. At the bottom , there is a box that shows the overall memory demand and the level of "Memory Pressure" the system is under.

This is important because operating systems use some of your storage space as "virtual memory" to supplement RAM. If your storage is full or near full, this additional memory will not be available, and the system will have to limit memory available to apps.

How much free storage space do you have on your internal drive?