One thing to check would be the speed of the USB adapter that came with your card i.e is it USB 2.0 or USB 3.0?
I don't know what the specs are of the SD card slot fitted to the Macbook Air, maybe someone here will.
Personally I would do a real world test, as this will truly answer your question with the type of video you record:
Shoot some video, maybe several segments, long short whatever you mostly do.
Without using iMovie, transfer, the video to your MacBook's hard drive - simply drag and drop it. iMovie will only convert your video and take longer, so don't worry about it for initial testing.
Time how long it takes using the USB adapter, and then do the same with the SD adapter.
This is a very simple method for determining which is fastest.
Another factor will be the speed of your microSD card.
Also you could try using the free program Blackmagic disk speed test (from the app store). This will test the speeds for you using the various connections.
I hope this helps.
