What is the fastest way to move in Genshin Impact?

Height matters

A hidden mechanic not really explained but something you can experience is that taller characters move faster than shorter characters. This is clearly evident when you compare someone like Diluc to someone like Qiqi. Bigger bodies mean the stride length is longer, so each "sprint" moves you further than shorties. Males in general are taller than female counterparts -- this is true especially with the Male/Female Traveler.

Theoretical Fastest Movement Speed

For the theoretical fastest movement speed, we need:

• 2 Ameno users in our party
• 4 party members from Mondstadt
• 1 party member who uses a R5 4★ Catalyst "Wine & Song"
• (Optional) 1 party member with sprint stamina reduction passive

Thus, with a party setup of Sucrose, Venti, Razor/Kaeya, Any other Mondstandt member, we can achieve a 10% + (5% * 4) = 30% movement speed increase.

Sources of Movement Speed Increase

• Impetuous Winds (Elemental Resonance)

  • Requires 2 Anemo users in your party
  • Increases Movement Speed +10%
  • Decreases Stamina Consumption +15%

• Constellation Lvl 6 for Amber

  • Fiery Rain increases Amber's Movement SPD by 15% for 10s.

• 5 ★★★★★ Catalyst "Lost Prayer to the Sacred Winds"

  • Increases Movement Speed +10%

• 5 ★★★★★ Sword "Skyward Blade"

  • Using an Elemental Burst increases Movement SPD by 10% for 12s.

• 4 ★★★★ Bow "Prototype Crescent"

  • Aimed Shot hits on weak points increase Movement SPD by 10% for 10s.

• 3 ★★★ Sword "Skyrider Sword"

  • Using an Elemental Burst grants a 12/15/18/21/24% Movement SPD increase for 12s.

• 3 ★★★ Catalyst "Twin Nephrite"

  • Defeating an enemy increases Movement SPD by 12/14/16/18/20% for 15s.

Bunny hopping (constant jumping) is actually faster than regular sprinting. Source: this reddit post where author did all necessary testing, direct video link from it.