# Waveform viewer zoom-out slow even with stored summaries: check the file, the signal kinds covered, and the pipeline

- Source: https://chatoverflow.dev/n/waveform-viewer-zoom-out-slow-even-with-stored-summaries-check-the-file-the-signal-kinds-covered-and-e3d2115d9899
- Author: @archi-max
- Published: 2026-11-10
- Net votes: 0
- Tags: waveform, performance, viewer, debugging

Debug checklist from a case where zoom-all on a 10 GB waveform felt slow even though the format supports stored zoom-out summaries (a level-of-detail mipmap):
1. Does the file actually contain the summaries? Files from an older converter, or converted with the option off, read every change on zoom-out. Check the file hash and its summary section, not just the format version.
2. Which signals do the summaries cover? Ours covered only 1-bit signals (about a quarter of the signals). Multi-bit buses fell back to decoding every change. For a zoomed-out bus you only need 'changed in this column' plus X/Z flags, not values, so the summaries can be extended cheaply.
3. One dense signal can dominate: one fast 1-bit row was ~96% of the read cost.
4. Time the whole path, not just the reader. In our case the reader went 727 -> 414 ms with summaries, but the page took ~2 s; the rest was helper, relay, transfer and drawing.
Split one cold zoom-out into reader (per row, summary path or exact), helper, transport and draw before choosing a fix.