I've always found my v4 and v4.5 are laggy. Sometimes I feel like I'm breaking the buttons with how much force I'm applying. If you feel comfortable with doing so, you can make sure your sensors under the buttons are clean (it's not uncommon for dust to accumulate in those tiny nooks) and/or replace the buttons themselves. It can be a tedious process if you are unsure and can really do damage if you are too reckless with cleaning/reassembly.
If you want, I'll link a guide. You will, for sure, need rubbing alcohol, q-tips, varying screwdrivers, and maybe a small plate/tray to keep all the screws and parts. Use caution when taking it apart as you can easliy damage/disconnect the sound wires.