Hello,
That should be fine if you do that locally in your codebase. Setting the spawnable actor label was only meant to be useful only in editor.
We noticed the same performance issues with setting actor labels, so we’ve actually addressed that for 5.6. You could potentially take this changelist into your codebase if you want the same fix. It introduces AActor::SetInitialActorLabel which allows the label to be set without incurring the cost of SetActorLabel.
https://github.com/EpicGames/UnrealEngine/commit/6134865e5adf90fedfa01cc0ce8f552f2aac0979