This is asked quite often and you'll find some answers in the existing threads:
TL;DR: Voukoder is only handeling the encoding part of the export. If Premiere is slow with rendering each frame before it's sending it to the encoder voukoder can't improve it.
Code
[19:38:25] Frame #520: vRender: 76980 us, vProcess: 7 us, vEncoding: 13975 us, aRender: 57 us, aEncoding: 1188 us, Latency: 92275 us
[19:38:25] Frame #521: vRender: 64296 us, vProcess: 9 us, vEncoding: 13036 us, aRender: 53 us, aEncoding: 1080 us, Latency: 78492 us
[19:38:25] Frame #522: vRender: 53933 us, vProcess: 6 us, vEncoding: 13430 us, aRender: 56 us, aEncoding: 1140 us, Latency: 68601 us
[19:38:25] Frame #523: vRender: 83936 us, vProcess: 5 us, vEncoding: 13734 us, aRender: 90 us, aEncoding: 1135 us, Latency: 98920 us
Premiere already needs ~80ms to render (vRender) a frame (decompressing the sources, filters, effects, ...). That limits the export to 12.5 fps. The GPU is pretty much ideling because it is waiting for frames from premiere.