Mac is running slow

Symptoms

If your Mac is running unusually slow
If your Mac is becoming unresponsive with a spinning wheel frequently
If your fans are spinning fast and Mac is getting hot frequently

Solutions

Soft Checks

Restart your Mac. This may clear any unclean processes or restart any rouge applications.
Close any open applications and reopen to test if the issue persists.
Check for any application updates pending.
Ensure your work environment is not blocking airflow to your Mac. No airflow or blocked fans will potentially overheat your Mac, and if running hot, will run slow to protect the device.

Check Activity Monitor

Note: Activity Monitor will show all open applications and their impact on the performance of your Mac. To open Activity Monitor, search it in Spotlight or find it in Applications > Utilities > Activity Monitor.

To find out what is taking up memory:

In Activity Monitor, click the Memory tab.
Review the Memory Pressure box at the bottom which shows the impact by your applications. If this is showing yellow or red, you may need to close some applications.
Review any applications listed to see if any applications can be closed if not being used. These will be shown in descending order of highest impact to lowest.
Click on those applications and click the crossed out stop sign in the top left corner to force quit a process.

Reset NVRAM

NVRAM (nonvolatile random-access memory) is a small amount of memory that your Mac uses to store certain settings and access them quickly. PRAM (Parameter RAM) stores similar information, and the steps for resetting NVRAM and PRAM are the same.

A step by step guide can be found here: TS1001 - Reset NVRAM

Reset SMC

Note: The SMC (System Management Controller) is responsible for these and other low-level functions on Intel-based Mac computers:

Responding to presses of the power button
Responding to the display lid opening and closing on Mac notebooks
Battery management
Thermal management
Ambient light sensing
Keyboard backlighting

A step by step guide can be found here: TS1000 - Reset SMC

Test in another account

Create another User on your Mac to test the OS environment.

Do this by:

Open System Preferences > Users & Groups
Click Unlock to make changes then enter your admin password
Click the + button on the left panel to create a new user
Fill the details for the test user account and click Create
Log out of your account, and log into the other account
Test with similar or same processes where possible to check if the same issue reoccurs