Speed

Speed, often abbreviated as SPD, is one of the stats in Honkai: Star Rail, used by both characters and enemies. In combat, it determines Action Value (AV), and consequently the order and frequency in which characters and enemies will act. Action Value can be set to visible in the Other Settings tab of the Settings menu.
\text{SPD} = \text{Base SPD} \times (1 + \text{SPD} \%) + \text{Flat SPD}
Base Action Value is the time between turns for each unit and is calculated based on their SPD:
\text{Base Action Value} = \frac{10000}{\text{SPD}}
Turn order is determined as follows:
# At the beginning of the battle, each unit's Action Value, representing the amount of time until that unit's next turn, is set to their Base Action Value.
# At the beginning of the battle, effects that take effect upon entering battle, including SPD/AV modifying effects, are triggered on characters from left to right.
# The unit with the lowest Action Value, i.e. shortest time until their next turn, is selected to be the next active unit.
# All units subtract that lowest Action Value from their current Action Value, representing the progress of time until the next active unit's turn.
# The active unit starts their turn. All DoTs on the active unit deal damage.
# If the active unit is player-controlled and Auto mode is not enabled, the player has the opportunity to cast any pre-action Ultimate(s). Pre-action Ultimates cannot advance the action of the active unit (e.g. by the set effect of Eagle of Twilight Line), since at this point the active unit's Action Value is already zero.
# The active unit acts, resetting their Action Value to their Base Action Value. The player can queue any post-action Ultimates during the action animation.
# Any post-action Ultimates queued during the action animation execute. Post-action Ultimates can advance the next action of the active unit, since at this point its Action Value has been reset.
# The active unit's turn ends, ticking down any Status Effects on the active unit. Some Status Effects only tick down if they were present at the beginning of the active unit's turn.
# Repeat from #3.
Over a long time, the number of turns a unit takes is proportional to their Speed.
==Cycles and Breakpoints==
Cycles are a mechanic found in certain combat challenges such as the Forgotten Hall. The first cycle at the start of each wave in a battle lasts for 150 AV. Once the first cycle has passed, subsequent cycles last for 100 AV each. This is the cause for many commonly-quoted breakpoints, along with some Planar Ornament set effect requirements.
SPD 	Breakpoint
114.3	4 actions in first 3 cycles
120 	3 actions in first 2 cyclesSpace Sealing StationFleet of the AgelessSprightly Vonwacq
133.4	2 actions in first cycle
135 	Firmament Frontline: Glamoth (first level)
142.9	5 actions in first 3 cycles
145 	Talia: Kingdom of Banditry
160 	4 actions in first 2 cyclesFirmament Frontline: Glamoth (second level)
171.5	6 actions in first 3 cycles
==Action Value Modification==
In combat, there are 2 ways to modify the current Action Value: Modifying SPD, or advancing/delaying action. The former changes the Base Action Value while the latter does not.
After a SPD modification, the current Action Value is affected as follows:
\text{Action Value}_\text{New} = \text{Action Value}_\text{Old} \times \frac{\text{SPD}_\text{Old}}{\text{SPD}_\text{New}}
In other words, the remaining time until the unit's next turn is prorated according to the speed change.
Action Advancement and Action Delay work as follows:
\text{Action Value}_\text{New} = \max \left(0, \text{Action Value}_\text{Old} - \text{Base Action Value} \times \left(\text{Action Advancement}\% - \text{Action Delay}\% \right) \right)
In other words, the time until the unit's next turn is advanced or delayed by a percentage of the unit's full turn time.
===Immediately Take Action===
Effects described to make a target "immediately take action" are different from those that "forward action by 100%." Such effects forcibly set the current Action Value of the target to 0, regardless of their Action Value prior to that. Therefore, "immediately take action" effects will still fully advance the target even if their Action Value is greater than their Base Action Value due to action delay effects, while "forward action by 100%" effects do not.
However, if an "immediately take action" is applied during another character's turn, the current character will still need to finish their turn first before the target who had their action advanced can take action.
==Status Effect Duration==
Some status effects unconditionally tick down at the end of the character's turn, while others only tick down if they were present at the start of that turn. All SPD modifying statuses fall into the latter category. For example, if uses her Ultimate in the middle of an ally's turn (including herself), the duration of Astral Blessing's SPD Boost will not decrease at the end of that ally's turn. The upshot is that using a SPD boost during the target character's turn gets the most out of its effect, whereas using it just before the target character's turn is the most wasteful.
==Enemy Speed==
Enemies have a predetermined Base SPD. Their SPD will also increase according to their level starting from Level 65.
Enemy Level	Base SPD Multiplier
1–64 	1
65–77	1.1
78–85	1.2
86+ 	1.32
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