Show duration instead of rate for slow progress

[kjgoodrick]

📝 Summary

When using the progress bar the duration (e.g. 2 min, 2s per iter) of each loop will be shown instead of the rate (e.g. 0.01 iter/s) when progress is slow.

🔍 Description of Changes

When progress is slow the current implementation of the progress bar is not very helpful as it is harder to interpret 0.01 iter/s than 1 min 40s per iter. When the rate is very low the rounding also makes 100s appear the same as 100m (both 0.01 iter/s). This commit detects when the rate drops below 1 iteration per second and switches the display to show time per iteration instead.

Comparison Table

Rate [iter/s]	Old	New
10	10 iter/s	10 iter/s
1	1 iter/s	1 iter/s
0.5	0.5 iter/s	2s per iter
0.3267	0.33 iter/s	3.06s per iter
0.0751	0.08 iter/s	13s per iter
0.009	0.01 iter/s	1m, 11s per iter
0.00165	0.01 iter/s	10m, 2s per iter

This uses the same humanizer that is used for the ETA, so the styles match. There is one small change to the humanizer: if the seconds are greater than 10, no decimal places are shown on the smaller unit, to avoid outputs like 10m, 2.04s.

📋 Checklist

• I have read the contributor guidelines.
• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• I have added tests for the changes made.
• I have run the code and verified that it works as expected.

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Project	Deployment	Preview	Comments	Updated (UTC)
marimo-docs	Ready	Preview	Comment	Nov 20, 2025 2:47am

[mscolnick]

nice, thank you for this change! its much more readable

[Copilot]

Pull Request Overview

This PR enhances the progress bar display by showing duration per iteration (e.g., "2 min, 2s per iter") instead of rate (e.g., "0.01 iter/s") when progress is slow (rate < 1 iter/s). This improves readability and prevents confusion from rounded rates that appear identical.

Key changes:

• Modified rate calculation to preserve precision for rates below 1 iter/s
• Updated frontend to display time-per-iteration format when rate < 1
• Adjusted prettyTime function to omit decimal places when seconds >= 10

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
marimo/_plugins/stateless/status/_progress.py	Modified _calculate_rate() to preserve full precision for rates < 1, updated docstring
frontend/src/plugins/layout/ProgressPlugin.tsx	Added conditional logic to display duration per iteration when rate < 1, modified prettyTime() to adjust decimal precision
tests/_plugins/stateless/status/test_progress.py	Added test_update_progress_slowly() to verify slow progress behavior with 12s sleep
examples/outputs/progress_bar.py	Added new example cell demonstrating slow progress with 12s delays
marimo/_smoke_tests/async_iterator.py	Added test_progress_slow_async() to test slow progress with varying durations

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[kjgoodrick]

Thank you both for the suggestions. I’ve mocked time.time to quickly test slow progress and added additional tests for prettyTime. I also refactored the prettyTime tests so each case reports its own failure, but I’m happy to revert that change if it’s not desired.

[akshayka]

Thanks!