Ki itself is light. Super Saiyan auras are made of
light energy and explicitly radiate like flames. Energy Waves are most commonly referred to as "
beams [of light]" (光線
kōsen) throughout supplemental material; in fact, Frieza's Death Beam specifically is described as a "
flash of light" (閃光
senkō) or as simply a "
light" (光
hikari) being emitted from his fingertip, which even Namek Saga high-tiers are incapable of defending against.
Ki Manipulation techniques can generate heat or even explode upon being absorbed by physical objects, like real world lasers can.
Ki itself can be diffused before being released, with auras and
Kiai being the most diffused forms of
Ki. The page for
Ki Manipulation even currently accepts
Light Manipulation as a sub-ability.
But also,
Ki is "
material energy converted from internal energy". And, also,
Ki can bend, and explode without being absorbed by solid matter, and can even create explosions without even being materialized, and can propel characters through free space, and can be compressed and given different shapes, and can be physically interacted with, and has tangible force. And again, if Frieza's Death Beam is accepted as real light and, thus, lightspeed, then the series hard-caps under lightspeed until after Post-Zenkai Vegeta and above.
But also, also, I think Piccolo's eye lasers are referred to as "heat rays" (熱光線 netsu kōsen) once, which could possibly be used to argue lightspeed as a form of thermal radiation.