I'd say that it's down to several factors, really.
Mainly, it's an art choice. Frieza's blast might seem slower because more panels have been interjected to make the scenario more impacting. The time it took from Attacker to Target is unknown, somewhat, as the blast needs to travel passed the supposed targets and to get their reactions. It's a really well drawn scene in which the blast is fired, and it goes down the line, intentionally missing each expected target and hitting Dende, allowing the shock of the situation to sink in.
Meanwhile, Piccolo destroying the moon isn't as exciting as one might expect. Not only is the moon appearing a lot closer than it should be to earth (That might also be an art decision, but seriously, look outside your window and see how small the moon is, and then compare it to how big it is when Piccolo destroys it. Either it's super close, or DBZ Earth's moon is enormous. Also, yes, this is nitpicking.) it's also a notably large target, and while it's never truly immobile, general gist is that it can't wilfully dodge and therefore, isn't that difficult to hit. Extra panels showing the beam travelling wastes time and paper, so the page only contains three panels to show what's happened.
Mainly, this tells us that the time between panels is not standardised, but we all knew this from the beginning. Also, it can be said that, potentially, a lot of feats and moments in DBZ could be attributed to art style and theme (but let's not open that can of worms.)
One other thing to consider is that you needn't be travelling at light speed to be undetected by the human eye. While you're not truly invisible until you move beyond light speed, simply being unseen requires much less speed to accomplish. Humans generally perceive the world at 1/24th of a second. If you're capable of moving faster than that, then you're likely to be unseen in the general human sense. DBZ made a big deal about it, with Piccollo explaining to Gohan that he needs to 'See with Ki' since traditional sight was useless while viewing the fight between Yamcha and the Saibaman. The Android Saga had a big impact here too, and it was one of the bigger advantages the Z Fighters had over Frieza, being that Frieza couldn't sense Ki. I would argue that, if this was well and truly the case, certain, major segments of the manga should have been wildly different, but, eh, not up to doing that right now.
Building on the previous point, Frieza and the Androids taking the group by surprise also builds on this Ki Sense thing. Since Frieza's power was entirely inborn, and the Androids didn't have Ki to manipulate, it's possible that their energy blasts were not made from Ki, and as such, they couldn't be sensed when fired. It's also possible that Ki Sensing doesn't just track Ki but simply amplifies your senses to the level of keeping up with everything else, but it would offer an explanation as to why the Z Fighters were unable to perceive an energy blast that was sub light-speed.
Make of that what you will, just thought I'd share my two-cents. If you'll excuse me, I'm going to go and grab some pain killers, as I feel that threads like these, no matter how well intentioned or written, will inevitably end badly.
Edit: Going back to the human perception thing, it DOES depend on the distance you are from the thing you're trying to see, and how big it is, but the rule still generally applies.
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