Honestly I don't really see... Is it because there's no explicit check for brush under the water?

Regardless if I know the "code to English translation", I still wouldn't know the solution 

My mod is actually lagging sometimes and I don't know what causes it??

It's not due to map brushwork.

All I know is it doesn't lag if I look away from roughly the center of the map

Is there any way for me to diagnose this cause, without having reverting my code line by line ?trial and error? 

*without line by line trial and error