Using the FNPRMS app link test I am seeing that Geckoview's loadURIDelegate is delay startup in Fenix by ~10%.

FNRPMS measures the duration from process start to pageload complete on example.com using logcat.
Navigation is triggered via intent sent by adb.

I also included a build where all of the Geckoview actors were removed, no actors. It appears that the loadURIdelegate and its cross-process messaging is the only impactful actor.

Each entry is n=25

baseline    wifi  1.371
baseline    wifi  1.376
no actors   wifi  1.239
no actors   wifi  1.235
no loadURI  wifi  1.241
no loadURI  wifi  1.233

baseline    no network  1.326
baseline    no network  1.311
no actors   no network  1.180
no actors   no network  1.184
no loadURI  no network  1.185
no loadURI  no network  1.182

These are local builds of GV and Fenix, so final results may shift somewhat.
(e.g. the latest fenix nightly from taskcluster has a startup time of 1.249 with no network).

The current plan to address this (as far as I know) is bug 1619796: not notifying the delegate for navigations that the app initiated.

I think this is a duplicate of Bug 1619796, after that is fixed we will not invoke the delegate anymore in this case (or any other case besides page-initiated navigations).

Yeah, this is a dup.