Steps you could try yourself to troubleshoot a slow Mac:

• Start by shutting down your computer. Time with a stopwatch the time that is needed from pressing the power button until the option to login. Also quickly check the time that is needed after login until you can start working. Define start working, for example when all icons appear in the menubar. When checking the time again use this as your reference.
• Identify what is slow, is the internet connection slow? Are the web pages appearing slow? Check and troubleshoot your internet connection. Clear Web browser cache.
• Check if your hard disk is full. A full hard disk slows down your computer. Cleanup your disk.
• Cleanup unwanted applications. Also cleanup unwanted web browsers plugins.
• Check for Mac OS X updates. Run those updates.
• Check and decide to update all applications to the newest versions. Also update Web browser plugins.
• Start Activity Monitor and check all processes with high utilisation and not dropping. Also check memory usage and availability . When you identify the software causing the problem. Stop it and check normal behaviour again. Search for an update for this software.
• Check if spotlight indexing is running. This could be the issue for slowing down the computer. When spotlight indexing stops check normal behaviour again.
• Check your antimalware software for correct behaviour. Check for updates available and check correct settings. It is possible you have to exclude files from on access scanning or choose a better antimalware program. Run a full scan and check for malware.
• Decide if you need Oracle Java or Adobe Flash plugin.
• Try running less applications at the same time and check normal behaviour.
• Check you Dock applications, options, open at login. Do they need to start  when you login?
• Start system preferences, Users & Groups, check login items. Decide if you want to automatically want to open when you log in. This setting also influences your login time.
• Clean up your desktop. To many items will slow down.
• Reduce transparency.  Go to system preferences, accessibility, display, choose reduce transparency. Check performance.
• Check the performance of FileVault Disk Encryption.
• Run Disk Utility and check for disk errors and volume errors.Beginning with OS X El Capitan, system file permissions are automatically protected. It's no longer necessary to verify or repair permissions with Disk Utility.
• Reset your System Management Controller (SMC). Check the Apple Support for the correct way.
• Check the hardware if your system can have more memory and the possibility for a SSD.