Stats: Improve scrolling performance

[staskus]

Fixes #22721

Majority of Stats Traffic / Stats Period scrolling hitches happen due to inefficient TopTotalsCell load, mainly calls to addRows which creates and adds multiple layers of UIStackViews every time the cell is configured.

Investigation: pc8eDl-1pe-p2

Solution

• Create and add UIStackView only once when the table view loads. When other cells reuse the same views, only configure or hide existing views.
• Use setNeedsLayout instead of layoutIfNeeded

This is only the first iteration of possible improvements. I didn't achieve a full smooth experience and animation hitches still happen after the first load. However, the improvements are measurable and visible, especially when testing on a debugger-connected iPhone.

Performance Comparisons

I decided to compare the performance of cell configuration methods by measuring how much time it takes to fully configure the cell. When testing, I use a high-traffic site and scroll up and down three times.

TopTotalsCell

iPhone 14 Pro

Condition	Performance Average (s)	Improvement Factor
Before	0.023552464503867953	-
After	0.005567020840115017	4.2x faster

CountriesCell

iPhone 14 Pro

Condition	Performance Average (s)	Improvement Factor
Before	0.02454930543899536	-
After	0.004590545381818499	5.3x faster

All Stats Cells Averages

The improvement of the configuration of all the Stats cell averages falls somewhere in between of two numbers.

iPhone 14 Pro

Condition	Performance Average (s)	Improvement Factor
Before	0.011049999985643613	-
After	0.0023589220599851747	4.68x faster

Hitches Comparisons

Before

After

Remaining Hitches

The majority of those remaining hitches are caused by issues that couldn't be identified or sometimes identified simply as main using Time Profiler:

To test:

1. Build Jetpack app on trunk with a debugger connected
2. Select higher-traffic site
3. Open Stats, select Days/Weeks/Months/Years tab
4. Scroll up and down a couple of times, and observe the performance
5. Build Jetpack app on task/22721-stats-traffic-improve-scrolling-performance with a debugger connected
6. Open Stats, select Days/Weeks/Months/Years tab
7. Scroll up and down a couple of times, and observe the performance
8. Confirm that cell appearance is not broken
9. Disconnect the debugger
10. Open Stats, select Days/Weeks/Months/Years tab
11. Scroll up and down a couple of times, and observe the performance
12. Confirm that cell appearance is not broken

Regression Notes

1. Potential unintended areas of impact

TopTotalsCell is only used within Stats, doesn't affect any other areas

2. What I did to test those areas of impact (or what existing automated tests I relied on)

Manual testing, and performance measurement

3. What automated tests I added (or what prevented me from doing so)

None

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr22847-2674401
	Version	24.6
	Bundle ID	org.wordpress.alpha
	Commit	2674401
	App Center Build	WPiOS - One-Offs #9460

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22847-2674401
	Version	24.6
	Bundle ID	com.jetpack.alpha
	Commit	2674401
	App Center Build	jetpack-installable-builds #8503

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.