Hi, I would go like so:
- Async Load Classes before spawning
(if that not already improved ur performance)
2. Store loaded Classes into an Array or Set
3. Burst-spawning an amount of actors into your Level until the Arrray is empty
Someone may has other solutions, it also depends how the BP are build by you in general.
For example:
- If your BP has some “Hard References” you will load them too at the same time…
- Event Ticks on many Actors with much code behind
- BP does something, while not even visible and it could be ignored
and so on… we would need more information on how and what you spawn 300 times at once 
and the amount of FPS drops (are they permanent low after spawning?)