Fix: Unity Physics Fixed Timestep Too Low Causing Lag

Game stutters under load. Profiler shows FixedUpdate running 4–6 times per Update. The physics rate is too high for what the budget allows; catch-up snowballs.

The Symptom

Frame rate drops cascade into worse drops. Heavy frames take 50ms+ because FixedUpdate catches up the gap, each step doing more work, frame stretches further.

The Fix

Project Settings → Time:
  Fixed Timestep:                  0.02      // 50Hz, comfortable
  Maximum Allowed Timestep:        0.1       // max catch-up = 5 steps
  Maximum Particle Timestep:       0.03
  Time Scale:                       1

50Hz physics is enough for most non-VR games. Bumping Maximum Allowed Timestep down prevents one bad frame from triggering many catch-up steps.

Move Work Out of FixedUpdate

Audit your FixedUpdate methods. Anything that’s not adding forces, calling Move on CharacterController, or running Physics queries should live in Update.

// Bad — runs N times per frame under load
void FixedUpdate()
{
    UpdateUI();           // 5ms
    ScanEnemies();        // 8ms
    rigidbody.AddForce(force);
}

// Good
void Update()
{
    UpdateUI();
    ScanEnemies();
}
void FixedUpdate()
{
    rigidbody.AddForce(force);
}

VR Exception

VR games typically run physics at 90Hz to match HMD refresh. Set Fixed Timestep = 0.0111. Watch CPU budget closely.

Verifying

Profiler → CPU Usage. Look at FixedUpdate.PhysicsFixedUpdate. Cumulative time should stay below 4–6ms per frame at target rate. Reduce work or rate until it does.

“50Hz default. Cap catch-up. Move heavy work to Update. Frame rate stabilizes.”

Related Issues

For Rigidbody2D interpolate, see interpolate. For Rigidbody sleep, see Rigidbody sleep.

Right rate. Bound catch-up. Lean FixedUpdate.