Q: What's the best fix for a slow computer?

What model computer? What are your primary uses and apps?

First thing anyone can do is replace or upgrade the system boot drive to SSD. Makes a very nice improvement.

How new or old is it? What version of OS X is it running?

Maybe it won't run faster but could run better with more RAM.

Take a look at all the tabs in Activity Monitor. If not on the Dock, go into Applications/Utilities and open it and keep it on the dock. Can tell you what is using your processor the most, RAM usage, disk and network I/O.

Don't install things that aren't needed. That can be counter intuitive to some but 'less is more' and some just really pull a computer system down. Security programs, "clean up programs" and others.

General purpose Mac troubleshooting guide:

Isolating issues in Mac OS X

Creating a temporary user to isolate user-specific problems:

Isolating an issue by using another user account

Identifying resource hogs and other tips:

Using Activity Monitor to read System Memory and determine how much RAM is being used

Starting the computer in "safe mode":

Mac OS X: What is Safe Boot, Safe Mode?

To identify potential hardware problems:

Apple Hardware Test

General Mac maintenance:

Tips to keep your Mac in top form