Svelte 5 Migration: Performance Gains, Cognitive Overhead

Upgrading a web application to Svelte 5 led to unexpected issues. Svelte 5's performance improvements, driven by "deep reactivity," introduce proxies and implicit component lifecycle state. While seemingly simpler, this adds abstractions, requiring developers to manage complex heuristics. Proxies aren't objects, and components aren't functions. The author details problems with proxies and callbacks, such as `DataCloneError` and props becoming undefined after component unmounting. Svelte 5 sacrifices developer agency, increasing cognitive load, leading the author to abandon it for new projects. The conclusion emphasizes choosing tools that leverage existing knowledge and promote understanding over cleverness.