---
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
