This solves ENG-10201, in a similar way to how D14136 (and associated stack) solved ENG-9952. The basic story is that creating too many Promises overwhelms Hermes, and in particular this occurs for many Farcaster-linked communities where all of the recent messages are membership robotext messages about users joining the community.

Luckily, we only need Promises for showInMessagePreview in reactionMessageSpec. This diff simply updates the signature of showInMessagePreview to allow for returning immediately (instead of a Promise), which solves the performance regression. More context on Linear here.

Deployed a Release build to my iOS device. Prior to this diff, it reproduced the cold start performance regression. Following this diff, it no longer reproduced.