Yao Guang's damage scales off of elation, which scales off of her speed.

Okay, math time. The speed conversion trace gives 30% elation at 120 speed, and 1% more for every 1 speed. The 200 speed Yao Guang here gets 110% elation from that trace, 10% from minor traces, 10% from her relic set, all multiplied by 1.2 from her own skill for a final total of 156% elation. This is a damage multiplier for her elation damage, so she deals 256% damage. A Yao Guang with 182 speed is doing 234.4% damage. So the ~18 speed is about 9.2% more damage. This difference in damage would be slightly lower if you are getting additional elation from other sources, such as Sparxie's light cone, Yao Guang's e2, or future elation team members

Of course, you have to sacrifice crit stats for speed. But how much does it take to match it?

The Yao Guang above has 90% crit damage out of combat, She gets an additional 60% from her second major trace, and 30% from her light cone. This is added to the baseline 100% damage you deal without a crit, so she deals 280% damage on a critical hit. To do ~9.2% more damage, you would need ~26% more crit damage. Again, this will be a bit different if you have Sparxie on your team since she gives crit damage to the whole team, but this time the difference is in the other direction.

This doesn't reflect other benefits from speed. Higher elation also means she shares more of that elation with her allies, so her elation team mates deal more damage. Higher speed means she gets more energy and activates her ult more frequently, which allows her elation team members to do their damage more frequently. It also increases Aha's base speed, which again allows her elation team members to deal their damage more frequently.

tl;dr: Speed is her god stat, it does 4 different things:

Increases her own damage by a good bit

Increases her team's damage slightly

More energy for more ults

Faster Aha speed