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 ...
This one was really difficult. Most of the traditional commentators have said that this verse refers to those studying Taittiriya Shakha.
But Kataka's commentary mentions that Kausalya had an independent Purohita by name Tittiri Maharshi. So, Taittiriya refers to his group and not the vedic shakha.
"तित्तिरिमहर्षिः कौसल्यायाः प्रत्येकपुरोहितः"
Scriptures mention a lot of contradictory things, it has been a lot of years since they were written and for the same reason the statements of scriptures contradict themselves.
For example Taittiriya (7.1.1) says :
 was created, the Vairaja Saman, of men the Çudra, of cattle the horse. Therefore the two, the horse and the Çudra, are dependent on others. ...
The other answer has the shloka attributed to Acharya Sayana. Sayana quotes it in two places - Once while commenting on Rig Veda 1.50.4 and again while commenting on Rig Veda 1.71.9. At both places Rig Veda mentions that movement for Surya is very fast. To establish that Surya indeed moves fast he quotes this verse.
It should be noted that whenever ...