After using my MacBook Pro for a while, it becomes very laggy (Swap usage high)

Solution 1:

Swap usage in itself does not mean that performance is impacted.

You seem to be judging swap usage as "high" based on the fact that 8.19 out of 9 GB are used - however the swap size is automatically expanded as needed. So there's no real "9 GB limit" or anything like that - so you cannot use this as a measure for saying that swap usage is either low or high.

Your performance is impacted when things are actively being swapped in or out while you're waiting for the computer to do something. Data being swapped in/out while you're not using the computer won't hurt the performance you see, just as data being swapped out and staying there won't hurt the perceived performance.