I would open the application "/Utilities/Activity Monitor" and look at the "Memory" segment.
1) look at the memory pressure highlighted at the bottom middle:
2) sort descending by the "Memory" column and not the highest memory users:
Note items whose User is you (my user on my computer is "wayne").
then try quitting those applications the usual way you quit applications. See if thing improve
If this does help you may want to consider adding more memory to your machine.
You can see your total memory in the "Physical Memory" list at the bottom