I just noticed that _createPreloadBrowser is called synchronously from _handleNewTab. We should probably run it asynchronously from there, or go even further and try to delay and batch it.

Not sure how much batching would improve in real-life scenarios (are there valid use cases of several tabs being opened at the same time?), but it would probably at least improve running mochitests. AFAICT this wouldn't cheat on performance or talos tests because the browser.newtab.preload is already disabled there.

Granted, _handleNewTab is (on most cases) running from a transitionend event, but even then it seems worth to at least give the event loop a moment to breathe before creating the next browser.