Exporting is taking an extremely long time (far longer than normal)

I'm trying to export 45 seconds of video but it is taking many hours to complete (doesn't look like it will ever export). I'm not using 3D fetaures, no effects, no motion blur etc - just a simple grid of short videos of a circle changing scale - that's it. I've tried refreshing everything, emptying cache folders, i've made sure the computer has enough meemory, no other programs running etc. I've exported this same video before with different colours and it took no time at all so I'm all out of ideas of what's gone wrong.

Dimensions of the video: 1920x1080

Each little dot is its own mp4 file - one video for each different colour.

30FPS

H.264 format

Here is the video: Single colour background. Grid of mp4 videos. Total length 45 seconds. 

Also, this video was originally made of vectors but that wou;dn't export so i'm now trying with mp4 files. Any help much appreciated.