https://u.gg/lol/champions/xayah/runes-table

I've played Xayah hundreds of games across 3 Diamond accounts trying both Lethal Tempo and Hail of Blades and found that Hail of Blades is much stronger overall unless the enemy team has 3 or more tanks.

Xayah's best trade pattern is getting a small amount of feathers down and landing the E root. Most of the time, enemies move out of 1 or both of the Q feathers before the first AA even comes out and Hail of Blades plus W helps fix this issue. The main reason Xayah is sitting below 50% win rate is because of her weak laning phase from taking Lethal Tempo and never being able to use it for trades.

In the current meta, it is rare for short and midrange ADCs to be able to stay in AA range for extended fights unless the fight is front-to-back and both sides have beefy frontlines (Lethal Tempo is better in this case).