the fan was still crazy loud in neutral

sounds like you have a bad clutch fan .Was your CEL on before your issues ?

fuel filter?

my filter was clogged and got the loss of power symtoms, slow rpm gain, no power

it threw no codes

Only concurrent symptom with the loss of power appears to be the engine fan on more than usual. That indicates high temperature. High temperature could be related to a failure of a catalytic converter. Symptom would be reduced power. Worth checking anyway.

Don't understand why OP would idle for 45 minutes. Not good for the engine.

Not exactly ideal as fan will run constantly, but not a problem as long as your cooling system is functioning. Why do you say not good?

OP - I'd guess you have a fan clutch issue after that long idle. I'm not familiar enough with the e53, but this isn't uncommon on some other chassis models. May be locked up and/or binding, which also explains the noise. The drag can lead to a feel of sluggish acceleration.

I'd start with the fan clutch. With the engine warm, you should be able to hold the fan still or slow it down greatly (use a cloth to rub lightly against the blade tips) with the engine idling. If it resists strongly and will not slow, the clutch is bad. An engine fan running at 4000 RPM can easily pull 15 horsepower off the engine which will slow your acceleration, but not necessarily diminish your cruising speed.

Beyond the fan clutch, I would look to a restriction in the air intake, then the fuel filter or fuel pump as the next possible issues. If any of these were bad though, it's unlikely you could reach maximum shift point RPM levels. Will the engine go over 4000 RPM on a full throttle upshift?

My experience with a bad catalytic converter on other vehicles was such a severe restriction in the exhaust system that the vehicle would not exceed 30 MPH, so I doubt that is your situation.

I suspect that your extended idle period encountered a failure point in your fan clutch. Good luck.

Because IMO you experience additional (and unnecessary) engine wear compared to shutting it off. Manufacturers know this, and those owner's manuals that list the conditions that qualify as Severe Service, thus requiring more frequent oil changes, include vehicles subject to extended idling. Idling means less efficient combustion, thus more unburned fuel. That results in more combustion chamber deposits. The exhaust catalyst will clean up the tailpipe, but that unburned fuel can wash down the cylinder walls, even with EFI. Some vehicles will have an idle speed increase circuit just to avoid extended low speed idle.

I think that engines are happiest when they are run at varying speeds, at varying loads. I have had to leave a vehicle idling for extended periods in extremely cold climate conditions, so you can't always avoid it, and at normal temperatures occasional idling isn't a problem. But extended idling of an engine designed to run up to 6000 rpm is bad practice, IMO.

Apart from the accelerated engine wear, why waste the fuel and pollute more? My idle limit is about 1 minute when parked.

In initial post Mad Mike writes that there is no overheating or smell associated with the loss of power. I keep coming back to the 45 minutes of idling and the sudden onset. I have never had a fan clutch drag an engine down that much and I have taken a clutch fan off cars in the past and replaced them with non-electric no clutch fan. Never had a transmission, maybe a torque convertor act like that. Per my post and JCL's I can't think of anything other than a cat.

Ok, we're on the same page then. It's just semantics. I take 'bad' to mean something that will cause harm to a car. I wouldn't consider normal wear and even sporadic periods of extended idling bad, guess I'd call it 'not ideal.'

Are clogged cats common on these things? In my experience they aren't on most, and very uncommon without first having O2 or related CELs. I agree though; while fan clutch can cause some drag, it's usually not as extreme as is described here. I've also run cars without mech. fans.