The speed of light is not directly from the Samhita. It is given by Sayana in commenting 1.50.4 verse of RigVeda. The devata of the verse is Surya. The verse is:
तरणिविश्वदर्शवो ज्योतिष्कृदसि सूर्य । विश्वमा भासि रोचनम् ।।
Swift and all beautiful art thou, O Surya, maker of the light, Illuming all the radiant realm.
As it talks about maker of light. ...
Here is how this journal paper translates it:
He (the Sacrificer, i.e. the king) offers the samgrahani ishti. (When he is about to offer that oblation, he says:) “I will keep this community together (under my authority).”
The paper also provides this explanation on what the verse is saying:
According to Apastamba 20.1.4, the Sacrificer offers the ...