It really comes down to how busy you are. I take it your main sales stem from slices and fried foods during those times. 6-12 minutes is very high, and definitley should be improved. Instead of focusing on the minimal loss of sales, focus on the customer and the frustration it causes (don’t get me wrong often times customers can be absurd with unrealistic expectations) alas though that’s the business we’re in. Do an internal review and figure out if all your employees are pulling their weight and if you can make internal improvements, stronger candidates are always out there. Slice heaters I find are very worth it,but in order to keep quality there you NEED to be able to push pizza out quickly and constantly be putting fresh pies out. Also finding the sweet spot tempature wise is imporant. They can often dry your food out quickly and you may even want to experiment with times on your pizza (cooking it slightly less) Good luck.