Premiere lagging?

there could be many reasons but you'll need to provide us with some info

Please tell us your system specs: OS version, Premiere version, amount of RAM, Hardware specs including graphics card

and You should check to make sure your system meets minimum hardware requirements

https://helpx.adobe.com/premiere-pro/system-requirements.html

and you need to tell us your source properties and your sequence settings.

and if by any chance some of your sources are from a screen recording program or a smartphone, the issue may be a variable frame rate.  post back if so and I'll give you that info, or just do some searching here and you'll find those answers.