Update your post to include full system hardware specs and OS information.
Include PSU: make, model, wattage, age, condition.
Disk drives: make, model, capacity, how full?
What specific programs and applications are lagging?
Use Resource Monitor and Task Manager to observe system performance. Use both tools but only one at a time.
Determine what system resources are being used, to what extent (%), and what is using any given resource when the lagging occurs.