A character's movement speed can be increased or reduced by a number of different skills and conditions. The default movement speed of a character moving forward corresponds to 288 game units per second.
Speed boosts or snares stack multiplicatively, meaning that you can multiply their effects to obtain the final speed. Movement speed cannot exceed +34% (1.340 speed) unless as a result of a single skill such as Dash, Pious Haste or Burning Speed. Movement Speed cannot be reduced below -50% (0.5 speed), unless as a result of a single effect, such as Mind Freeze. Note that the sideways or backwards movement penalty is applied after the cap. Many NPCs (particularly Hard Mode enemies) have an inherent speed modifier, applied after the cap.
Effects that reduce speed by more than 50% (typically Water Magic hexes) also nullify all speed boosts. For example, if you are hexed with Mind Freeze, you will move at 10% speed, even if you use a BU, rainbow rocks, a cupcake, and Dash.
With an Essence of Celerity and a Birthday Cupcake, your speed should be 1.2 x 1.25 = 1.5. However, it's capped at 1.34, 34% faster than base movement speed.
If you get Crippled, your speed is reduced by 50% to 1.2 x 1.25 x 0.5 = 0.75. This doesn't exceed the cap, so your actual movement speed is 75%.
Adding *rainbow rocks* should increase your speed to 1.2 x 1.25 x 0.5 x 1.15 x 1.25 x 1.33 = 1.43390625; once again, it's capped at 1.34.
List of common effects
|Effect or Skill||Modifier||speed factor|
|Tar Effect (UW Pools)||-30%||0.7|
|Essence of Celerity||+20%||1.2|
|Red Rock Candy||+33%||1.33|
|Blue Rock Candy||+25%||1.25|
|Green Rock Candy||+15%||1.15|
|Shadow of Haste||+15%||1.15|