Are there performance implications using the "timestamp" property of ImportMulti in the following way?
importmulti finds the earliest of the timestamps, and rescans from that point after all imports are done, yes. The wallet stores metadata about keys including the timestamp, which is why a timestamp is provided individually for each address rather than a single timestamp overall. Other commands like dumpwallet export a timestamp per key as well so this makes sense to provide it per-key when importing too, to save you having to look for the earliest before passing them in.