Which Mac do you have?
How much RAM is installed?
Do you have a spinning hard drive - or an SSD?
How long have you been using it with Sierra?
A new upgrade to Sierra can take several hours of use before the system settles in.
You may be able to tell in your Activity Monitor, as it will likely show that services, such as mdworker, may be taking a lot of resources. This is quite normal for the first few hours after an upgrade.
If you have just upgraded to Sierra, give it a day or two.
If you have a spinning hard drive, instead of an SSD, then it may not get much better.