From canary it looks like we've found a significant "leak" in install latency. Shortening the time until we start querying the package manager whether the webapk install has completed reduces median and 75th install duration by as much as 4 seconds:
https://uma.googleplex.com/p/chrome/timeline_v2/?sid=a9045423875184a84be7b3a20888b445 

This was the CL that did it:
https://chrome-internal-review.googlesource.com/c/clank/internal/apps/+/430828

It's a single character change (changes the timing from 15seconds->5 seconds) and is only triggered when user tries to install a WebApk (low-risk). I know we're late in m61 but was hoping it can be considered for merge as the risk is entirely restricted to the feature at hand and is a noticeable improvement for user experience.