I will assume that Maxi-flo is a 260,000 BTU heater based on the model number. With the weight of water at 8.34 Lbs/gal and a 30,000 gal pool there is roughly 250,000 lbs of water to heat. Under ideal conditions you could see up to a 1 deg/hr rise. However, the 260,000 BTU is the input rating and most older heaters run around 70%-80% efficiency so that put the output just under 210,000BTU. Still not bad but now puts the temp rise at .83 deg/hr. Without a cover to help with heat loss I can see how you will be around 1 deg/2 hr. Something to keep in mind here. If you had, say, a 520,000 BTU heater and conditions the same, the pool would have heated to the same temp in half the time but, with the same fuel usage. Again, this is not taking into consideration all the extra factors such as wind, humidity, ambient air temp etc. which all affect the ability for a heater to heat and a pool to retain heat