FixedUpdate vs Update in Unity - Performance Comparison

Performance: FixedUpdate vs Update

MEDIUM IMPACT

This concept affects the smoothness and responsiveness of game frame rendering and physics calculations.

Updating physics calculations in a Unity game

Unity

void FixedUpdate() {
    // Physics movement code here
    rb.AddForce(force);
}

FixedUpdate runs at a fixed timestep, ensuring consistent physics calculations and smooth motion.

📈 Performance GainStable physics simulation with consistent frame pacing improves perceived smoothness.

Updating physics calculations in a Unity game

Unity

void Update() {
    // Physics movement code here
    rb.AddForce(force);
}

Physics updates in Update cause inconsistent physics steps because Update runs at variable frame rates.

📉 Performance CostCauses jittery physics and uneven frame pacing, leading to poor user experience.

Performance Comparison

Pattern	Frequency	Consistency	Input Responsiveness	Verdict
Physics in Update	Variable (per frame)	Inconsistent physics steps	Good	[X] Bad
Physics in FixedUpdate	Fixed timestep	Consistent physics steps	N/A	[OK] Good
Input in FixedUpdate	Fixed timestep	May miss inputs	Poor	[X] Bad
Input in Update	Variable (per frame)	Captures all inputs	Excellent	[OK] Good

Rendering Pipeline

Unity's rendering pipeline separates physics updates (FixedUpdate) from frame rendering (Update). FixedUpdate runs at fixed intervals for physics calculations, while Update runs every frame for rendering and input.

→Physics Simulation

→Game Logic Update

→Rendering

⚠️ BottleneckRunning physics in Update causes inconsistent physics steps, leading to jitter and uneven frame pacing.

Optimization Tips

1Use FixedUpdate for physics to keep simulation stable and consistent.

2Use Update for input and rendering to ensure responsiveness and smooth visuals.

3Avoid mixing physics code in Update to prevent jitter and uneven frame pacing.

Performance Quiz - 3 Questions

Why should physics updates be done in FixedUpdate instead of Update?

A Because FixedUpdate runs at a fixed timestep ensuring consistent physics simulation

B Because FixedUpdate runs every frame for smooth rendering

C Because Update cannot access physics components

D Because FixedUpdate is faster than Update

DevTools: Unity Profiler

How to check: Open Unity Profiler, record gameplay, and check CPU usage for FixedUpdate and Update calls. Look for spikes or inconsistent timings.

What to look for: Consistent FixedUpdate timings indicate stable physics; irregular Update timings may affect input responsiveness.