Try this it worked for me. In Bandicam you can set it in the video settings to stop recording the file every 4 minutes or even every minute or whatever you want. In other words it records the file but after let's say, you set it for 5 minutes, it will stop recording that file and keep recording making another video file. I have an external usb drive and noticed it was lagging a bit but I figured out that the files bandicam were saving were too big for my external usb drive so when I did that setting it quit lagging! Go to youtube videos on this forum and look at the videos I recorded by criticofgames You'll see it doesn't lag and I do giant videos.
So basically try that...set your videos to stop recording like every 5 minutes so if you are recording a 15 minute long video it stops at 5 minutes then keeps recording the next 5 minutes then stops at the end of 5 minutes then keeps recording until you stop. Then you'll have 3 videos but they are smaller size and you just edit them and put them in your video editor even windows movie maker that comes with windows does it. Try it, for sure it will make the game less laggy just experiment with file sizes.