Optimize frontend with memoized FeedItem and efficient IntersectionObserver

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+# Performance Analysis & Recommendations
+
+## Executive Summary
+You are correct that the React implementation introduces inherent latency compared to the vanilla JS/Backbone version, particularly during initial load and interactions. While backend measurements show fast response times (~5ms) for the current dataset (where `full_content` is mostly empty), the React frontend's rendering strategy and bundle overhead are the primary contributors to the perceived slowness.
+
+## Key Findings
+
+### 1. React Rendering Overhead vs. Vanilla JS
+- **Initial Load**: The React application requires downloading a larger bundle (~241KB vs ~150KB for legacy) and then executing hydration before fetching data. This adds an initial delay (Time to Interactive) not present in the lightweight vanilla version.
+- **Excessive Re-rendering**: The current `FeedItems` component triggers a re-render of the **entire list** (and all children `FeedItem` components) whenever a single item is selected or modified. React's reconciliation process diffs every item, which is computationally expensive compared to Backbone's direct DOM manipulation of a single element.
+- **DOM Node Count**: Even with small content, rendering 15+ complex article cards with `dangerouslySetInnerHTML` involves significant DOM operations. React's synthetic event system and component overhead add micro-latency per item.
+
+### 2. Implementation Inefficiencies
+- **Observer Churn**: The `IntersectionObserver` in `FeedItems.tsx` disconnects and reconnects for *every item* whenever the list updates or state changes. This is an O(N) operation that degrades performance as the list grows.
+- **Lack of Memoization**: `FeedItem` components are not memoized (`React.memo`), causing them to re-render unnecessarily on parent state changes (like selecting an item with `j`/`k`).
+
+### 3. Backend Data Strategy (Minor Current Impact, Major Risk)
+- Although `full_content` is currently empty for most items, the backend still retrieves and serializes this field for every item in the list. This is "fast enough" now (5ms) but remains a structural inefficiency that will cause severe slowdowns if/when content is scraped.
+
+## Recommendations
+
+### 1. Optimize React Rendering (High Impact)
+The most effective way to restore "snappiness" is to reduce React's work:
+- **Memoize `FeedItem`**: Wrap the component in `React.memo` so it only re-renders when its specific props change. This prevents the entire list from flashing when you select one item.
+- **Virtualize Long Lists**: Implement `react-window` or `virtuoso` for both the sidebar (`FeedList`) and main view (`FeedItems`). This ensures only visible items are in the DOM, keeping the browser responsive regardless of list size.
+- **Stable References**: Use `useCallback` for event handlers passed to children to prevent breaking memoization.
+
+### 2. Fix Observer & Effect Logic (Medium Impact)
+- Refactor the `IntersectionObserver` in `FeedItems.tsx` to maintain a stable observer instance using `useRef` and only observe/unobserve specific elements as needed, rather than resetting the whole list.
+
+### 3. Backend Optimization (Defensive)
+- Even if not the current bottleneck, modifying `item.Filter` to exclude `full_content` on list views is a simple change that prevents future performance regressions.
+
+## Conclusion
+The "sluggishness" is primarily due to React's re-rendering of the entire list on interactions and the initial hydration cost. Implementing **Memoization** and **Virtualization** will bring the responsiveness much closer to the vanilla JS experience.