From: Team Rocket Elite | #055
Marisa is slightly slower than lightspeed. If she was much slower, she wouldn't be able to fire the second attack before the first hit.
Well the scans are there for people to interpret. Marisa is talking about lightspeed before showing off an light based attack that is acknowledged by her opponent to be a light based attack. I feel that reasonably implies that the attack really is moving at lightspeed even if there isn't an explicit statement from a impartial source to clock in the true speed of the attack.
There is no indication of that. Speed is simply measure by distance covered over time. An object only moving half the speed can still do it provides it moves half or less the distance as the beam before sending the 2nd attack.
I assume you're talking about this page:
In which case it's pretty obvious she's be facetious and condescending.
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Deth Pen