Even though your laptop has decent hardware specifications, the flawless gaming experience depends on the system requirements of the particular game you are playing.
Assuming that all the games that you play on the laptop start lagging after sometime, the possibilities are that your Windows might be running out of virtual memory. You can manually increase the virtual memory (pagefile size) of your operating system up to four times of the total physical memory (RAM) of your laptop. (For example: For 8 GB RAM, you can increase your virtual memory up to 32 GB.).
You can find step-by-step instructions on managing virtual memory here.
In addition to this, you may also try disabling your antivirus if it is resource intensive. (Unlikely)
If all the above solutions fail, as a last resort you can install any third-party game booster such as GameFire, Razer Game Booster, MSI Afterburner, etc. to improve the game performance.