Wood Burning Pool Heater (55 Gallon Drum) finally running. Some Questions?

Finally got my 55 gallon drum heater in and working. So far so good, with a small exception.

Right out I didn't pay attention to my pump timer, and during full heat the pump shut off. When it clicked on, it was exciting to say the least. Some leaks to repair but got that all done, and have some new stuff in place to help prevent it.

This is the kit you've probably all seen from Youtube videos, the only difference is that I didn't like the use of soft copper 1/2" coils. That pipe diameter is very restrictive, and since I plumb most days for work, the soldering wasn't an issue. So there are 6 - 12' length of 3/4" copper in there, 72'. The bonus is the water moves through slower (if I want) and there is less backpressure on it. A surprise benefit was that where I had the tee off my jet to feed this, I had put valves to restrict the jet and force water through the heater. I found on High for my pump though, with no diversion it flows 5-6 GPM through the heater anyways, which is great. On low I have to divert it about 80% to maintain that.

I put in a Grunfos style circulation pump to see what would happen, it works, but flows slow without the pressure, and even though its stainless, the bearing is lubricated by the water so I doubt I'd get more than a season out of it. I also put a tee in ( all have valves to control the flow ) so when I top the pool up, I can use the Gardenia fitting to put the hose on and fill the pool through the heater so I'm not putting 8C water into the pool. I also use that same point to drain it after rains.

SO, my questions are with the science of heating. Is there any net benefit to running the water through it faster, but the outflow temp is lower, or slower so the outflow temp is higher, but less turnover?

Can't wait for warm swimming, we went in last Thursday when it was 26C, and the outflow from the heater makes a nice warm spot you can swim in. The heater yesterday was able to raise the temp 1C in 5 hours, but that was without out cover on, and I tried it from 5pm to 10pm, when the temperature here went from 20C at 5pm to 13C at 10pm. Hopefully out summer is finally starting so this week I'll get to see how it can do with a cover on nights and higher night and days temps for the next few days.

I have put a proper chimney and pex pipe about 3'down into the pool.

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While I don't get all the plumbing (amazing job!) I can tell you that generally, the faster you move the water through the system the better. It's not about temperature, it's about BTUs /KCalories. 1000 gallons 1 deg warmer will add far more heat than a teapot full of boiling water. Usually, the limiting factor is the pressure the plumbing can stand without leaking. This mistake is frequently made with solar heating systems--people assume their panels have to get the water good and hot and then trickle in. That's wrong. When I split my solar heating system into two separate systems, nearly doubling my water flow, my pool heated up far faster, despite the fact that my surface area is unchanged. Why? Because the maximum flow through the panels could be effectively doubled. You've actually done that with your 3/4" plumbing in the heater.

Think of it in reverse: Would your car radiator be more efficient at cooling your engine if the coolant flowed slower or faster?

Heat energy, not temperature, is always the key in heating systems.