Playing at 140 fps, yet recording at 15

[PigeonHudson]

I play games at 140 fps. Specifically, League of Legends, maximum graphics.
When I hit record on Dxtory, 1080p, it records 15 fps even though I'm still playing on 140.
If I drop it to 720p, I can hit 30 frames recording.
Why is this?

I have an AMD R9 270x and an i7 3.4 GHz processor. I've seen people upload 1080p videos of League on a far less powerful PC. So I assume it isnt a component issue.

I am using the Lagarith Lossless Codec fyi.

[ExKoder]

Please submit a Environment Information.

http://forum.exkode.com/index.php?topic=2.0