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+---
+epic: 13
+story: 13.1
+title: "Real-Time Evaluation (< 16 ms per Line)"
+status: draft
+---
+
+## Epic 13 — Performance & Reliability
+**Goal:** Instant evaluation, never lose work.
+
+### Story 13.1: Real-Time Evaluation (< 16 ms per Line)
+
+As a CalcPad user,
+I want every line to evaluate in under 16 ms so that a 500-line sheet completes in under 100 ms,
+So that calculations feel instantaneous and I never experience lag while typing.
+
+**Acceptance Criteria:**
+
+**Given** a sheet containing 500 lines of mixed arithmetic, unit conversions, and variable references
+**When** the engine evaluates the entire sheet
+**Then** total evaluation time is under 100 ms on an Apple M1 or equivalent modern x86 processor
+**And** no individual line takes longer than 16 ms to evaluate
+
+**Given** a user is typing on a line that triggers re-evaluation
+**When** a keystroke modifies an expression
+**Then** the result column updates within one animation frame (< 16 ms) for that line
+**And** the UI thread is never blocked long enough to drop a frame
+
+**Given** a sheet with 1 000 lines of complex expressions including nested unit conversions and currency lookups
+**When** the engine evaluates the full sheet
+**Then** total evaluation completes in under 250 ms
+**And** a performance warning is logged if any single line exceeds 16 ms