Most efficient way to get completed flights for list of planes

I'm trying to get data on completed flights for a list of about 300 planes, and I'm just figuring out how to use the REST API, using it with Python requests.
    
If I request data using api/states/all, I can include the "icao24" for multiple planes. However, when I use api/flights/aircraft to get data on completed flights, it seems to only work when I use one "icao24" at a time. And when I then iterate through my list of "icao24" making individual requests, there are sporadic delays of several seconds or even longer in the API returning data. I am including my username and password in my requests. 
    
Grateful for any comments about what I'm doing wrong, or what I might do better.

Thanks!

The API is a great resource. I'm having fun exploring it. Tks for the quick reply to my question.