I find the middle button is generally the first to fail.
They tend to degrade over time, missing a click now and then, versus abruptly stop working.
Try a new mouse.
Several things may cause laggy model performance.
SketchUp relies heavily on the graphics card, its driver and the single core speed of the CPU.
It helps us help you if we know what hardware you’re working with.
Kindly fill in the missing hardware information in your profile.
Modeling technique has a tremendous impact upon model performance.
No amount of fancy hardware can compensate for poor modeling technique.
See this compendium of performance modeling techniques…
How do I make SketchUp Run Faster? — SketchUp Sage Site