I specifically made this build to challenge the purpose of how exactly Lethal Tempo needs to be used to make it a good keystone. Achieving a massive attack speed over the 2.5 attks/sec limit is something that many people at almost all levels have come to overvalue, so my goal was to make a build that utilized a lot of the rune while still letting Ashe have her own aspects that make her a high utility ADC. Namely, high CDR for playmaking, and while having decent potential to kite, heal, and deal sustained damage at pretty reasonably high numbers.

This build probably won't outdamage all the glass cannon hyper carries, but it may outplay and outsustain them. Rough stat numbers are provided below

Keystone: Lethal tempo. At level 1 this keystone gives the attack speed of a rapidfire cannon. By using long range poke Volley or Arrow, Ashe can proc the rune and initiate with good synergy. This Keystone also lets one throw out the second zeal item while maintaining the same attack speed levels in team fights that she would have had with 2 zeals in season 7, so i cut the second zeal item out. This frees up one space in the item build that can be used for Essence reaver, arguably the most crucial item in the build.

Main tree runes: In games where being cc chained to oblivion isn't a major threat, i choose Triumph and Legend: Bloodline to help boost survivability through out the game, and coup de grace as a finisher if the enemy team isn't excessively tanky. Against heavy cc, Legend: Tenacity maxes at 30% tenacity

Secondary Tree: Any tree gives a bonus 18% attack speed which replaces the attack speed runes most ADCs used in the last season. I chose the ultimate hat since this build will have a 40% cdr, Ashe Arrow at a 55% CDR gives her near constant playmaking potential. If playmaking is not a big concern, Manaband Flow may help with any early game mana problems instead. Second rune, transcendence is second only to lethal tempo in terms of important runes for this build. At 10 minute you gain 10% CDR, and overcapped CDR becomes 1.2 AD per 1% CDR, this build will likely overcap 20% CDR for an additional 24 AD onto an already heavily stacked AD build, while Lethal Tempo maintains the heavy attack speeds expected of ADCs.

Items are all situational. If everything goes perfectly well (it probably won't), Essence reaver rush, zerker greaves and runaan's max out CDR, provide heavy damage, sustainable mana, etc. Followed up with IE to increase damage and crit chance for heavy teamfight damage, and black cleaver for tank shred and 20% CDR converted to 24AD. Lastly BoRK for crazy damage, and, with Legend: Bloodline, 20% lifesteal (same as BT, more damage) and attack speed.

Everyone wants to roll the enemy team easy, but if things go awry, certain item combinations work well together:

Merc Treads and Mercurial Scimitar can minimize cc and provide good MR
Ninja Tabi and Death's Dance will help nullify heavy AD burst or sustained ADC damage by way of passive effects, while also giving 92 attack damage because of the CDR overcap.

This last one should be avoided if possible since it gives transcendence max value: If the enemy team has a heavy healing champ like Vlad or Soraka, replace cleaver with Mortal Reminder. You gain grievous wounds, but lose cleaver's move speed buff and hp that helps kite and sustain.

Attack speed: Given the ideal build, the items provide (40+35+25)% attack speed = 100% attack speed (plus 80% while Tempo is active, and 40%? while rangers focus is up, and 18% from secondary runes gives 238% which should make her Attack speed ~2-2.2).

Bonus AD: Ideal build has 220 bonus attack from items and 24 from CDR overcap, for about ~300-340 total AD.

Total Stats: 300-340 AD, 70% crit chance at 2-2.2 attack speed with 20% lifesteal and 40% CDR.

Against Heavy AP Build:

Attack Speed: This build removes BoRK and Zerker's Greaves for 178% attack speed with runes and skills active, making it about 1.8 attacks per second.

Bonus AD: Mercurial swapped for BoRK gives 25 more AD so 269 Bonus AD for a total of ~325-365

Total Stats: 325-365 AD at 1.8 attacks per second with 18% lifesteal and 40% CDR

Against Heavy AD Build:

Attack speed: this is the same as the one above: 178% attack speed bonus for 1.8 attacks per second

Bonus AD: BoRK trades out for Death's Dance adding 40 damage plus 12 from CDR giving 296 bonus AD totalling ~352-392 total AD.

Total stats: 352-392 AD at 1.8 attacks per second with 23% lifesteal and 40% CDR.