I have now been hit with the Shimano cracked crank - but i guess i can’t complain - 7 years and nearing 50k miles. My question is this - I’m convinced it’s been cracked for a while now looking back and have had difficulties keeping in with my group ride lately that i usually can keep up with. I’ve been so bummed with my sudden seemingly decreased fitness and thought maybe something was off physically. I’m wondering if that cracked crank (on several of the spindles) could have caused some of this? Or am i just sucking more and more. 
Hard to say anything without more info.
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What would you estimate is your percentage drop in performance? Is it related to particular duration and/or power?
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What is the current state of your crank? How bad is the crack failure and where exactly is yours failing?
Basically, unless the crank was flopping around notably enough to see and feel, small flex via potential separation isn’t likely to lead to a massive drop in power delivery. Presumably the failure is on one crank arm and small enough to go unnoticed by feel, it’s not draining power significantly.
If possible, get an immediate crank replacement and ride again. If you still have the loss of failure, it sure ain’t the crank. If you find all sorts of juice, it might be related to the failure.
It’s on at least two of spindles towards the center, is flexing pretty good. It’s why i stopped-i thought something was wrong with my pedal/cleat/shoe.
Yeah, at that point, there could be some notable flex and loss. Depending on how long it’s felt like that, you might have suffered for that duration.
In short, no one will really know for sure or be able to tell you when/what losses were present. Fixing it and moving forward is the real direction to take IMO. It’s a bit like finding out a power meter was giving data. Unless you can pinpoint the precise moment of deviation, you are guessing at the timing and impact.
Yup… new crank ordered. I’m hoping that caused some drop. Otherwise i remain frustrated with my declining abilities. 
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I would complain. Cranks shouldn’t fail like that. I can’t believe you’ve been riding it cracked. They can fail catastrophically causing you to crash.
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Hard to determine, as mentioned by @mcneese.chad.
The thing about flex/play is that it “can” be power-robbing but isn’t necessarily so. An undamped spring takes force to compress but there’s minimal loss as it bounces back.
The Q-factor of a damped system (not to be confused with the Q-factor as it relates to pedal stance), determines how much energy is lost.
A cracked/debonded crank sucks. There may or may not be a silver lining here, but you’ll know soon enough.
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On my admittedly much older XTR cranks the line of your cracks is on appearance where the outer chain ring bolts onto the spider. I.e where the spider ends and the chain ring starts. Are you sure it’s actually cracked and not just loose (or broken) chain ring bolts allowing movement?
For instance here’s your actual outer chain ring. You can see that your supposed cracks are where the chain ring naturally ends.
