Fenix startup time increases with the number of open tabs

[hawkinsw]

Why/User Benefit/User Problem

The more tabs that a user carries over between browser sessions, the longer it takes to start Fenix. As evidence of this behavior, consider the following two graphs:

Along the y axis are start times and along the x axis are the number of open tabs restored from a previous browser session.

Impact

For a user who keeps a large number of open tabs, this means an unacceptable user experience.

Acceptance Criteria (how do I know when I’m done?)

Startup time will remain constant no matter how many open tabs are carried over between sessions.

┆Issue is synchronized with this Jira Task

[mcomella]

Triage: we need to know how many tabs users keep open to prioritize. @ecsmyth is getting numbers for this in #7191 (comment)

[mcomella]

Action item: determine how bad this is on the G5 (e.g. by taking a run with no tabs, 10 tabs, 20 tabs, 50 tabs, 100 tabs and comparing). If it's bad, consult team (ecsmyth?) and prioritize accordingly.

[mcomella]

Note: tabs are not currently instrumented in fenix startup so, assuming it has no impact on Displayed time, we may not be able to measure until #7781 lands – in our experience, top sites will always load after tabs so by instrumented top sites, we'll be able to observe the difference on tabs from "fully drawn" time

[MarcLeclair]

As of early March, with all the changes we've had, this is what I've noticed on a G5+ with 10 runs for each # of tabs:

As of now, I think we should focus on the weird spike that can be seen when the first tab is opened.