smaller batches in large coolers lose more heat. Throw a sheet of foil on top of the grist, wrap it up good with blankets, start a touch higher than you want, and leave it alone. Don't open it to stir, take temps, etc. Trust me, it doesn't affect efficiency if you don't stir in the middle of a mash. That's more about lautering, or freeing up the converted sugars after they're converted. Extra blankets, leave it be, start a bit high, use more water-to-grist to get total volume up (I go up to nearly 2qt/lb).
Start cooling your samples if you're using a hydrometer. Your OG should be higher than pre-boil since you lost water but not sugar.