Macs can start to seem slower for a variety of reasons. Some of them you can't really do a lot about, such as installing the latest operating system on very old hardware, but there are a few things you can do to make sure everything is running as quickly as possible. Here's some things to try:
1) Ditch the drive. If you've got an older Mac with a spinning hard drive, it's like having a speed boat with an anchor overboard. Hard drives have been around since the 50s, and although they've gotten smaller and hold more, they're still the slowest part of any computer. Replacing your hard drive with an SSD is like dumping the anchor and adding a jet engine to your speed boat.
You can find out what kind of drive you have by going to the Apple menu and choosing About This Mac, then clicking Storage. If it doesn't say "Solid State" under SATA Drive, then you could benefit.
Difficulty: Moderate. Cost: Moderate depending on size.
2) Add RAM (memory). If your machine has 8 GB of RAM or less, see if you can add more. This can make a big difference if you tend to have more than a few apps open at a time.
To find out whether you can upgrade the RAM in your Mac, download and run the System Scanner from Crucial. It'll tell you how much RAM you have, how much your machine will hold, and even let you order more RAM right from the page.
Difficulty: Easy, assuming your machine can be upgraded (the latest ones often can't!). Cost: Low.
3) Try a new user. If you're the kind of person who likes to experiment with software you might have stuff installed on your system that's slowing it down. Sometimes signing in as a new user can help reveal these situations. If you log in as another user and things seem much snappier, then it may be worth setting your user up again from scratch. Here's how to create a new user.
Difficulty: Easy (unless you choose to rebuild your user!). Cost: None!
4) Run EtreCheck. Have you migrated from one Mac to another to another to another ad infinitum? If so, you've likely got a lot of "cruft" in your system that serves no purpose and could be causing issues from slowdowns to crashes. EtreCheck is a program that will scan everything installed in your system so you can help identify stuff you don't need anymore. If you register the software it'll even make it easy for you to delete it. It'll even give you information on how well your machine is running and what you can do to improve things.
Difficulty: Easy. Cost: Free to run, $20 to purchase
5) Reboot. The Mac is amazingly stable. I've seen them go for months without needing to be rebooted, but rebooting your Mac is like getting a good night's sleep—it gives the machine a chance to do some simple maintenance and start with a clean slate.
Difficulty: Extremely difficult (just kidding). Cost: None!