Unexpectedly long export times?

Hello, 

I'm experiencing a perplexing issue with exporting a portion of a video I recorded with OBS, and I'm not particularly sure what the problem is. The original file is an mp4 that's a little over an hour long, and is recorded at 1080p. (Windows explorer is telling me the source file's bitrate is roughly 85569Kbps.)

I'm trying to render out a 12 minute clip as H.264, at 720p, with a CBR of 10Mbps to keep file size at roughly 1GB. My issue is that the render time, regardless of whether I export directly, or use the media encoder is 45 to 50 minutes long. I'm using software rendering with an i7-8700k and 24GB of RAM, I have the files on my SSD... Although my processor never goes above 50% utilization during the render. I made sure render at maximum depth is not on to try and speed up render times.

Is the issue purely because the bitrate of the original file is so much higher? I feel like the amount of time to render out such a small video is kind of ridiculous. Am I missing something here? I have noticed over the life of this computer (a little over a year) render times have been increasing. Previously I rendered out an entire hour long video in with roughly ten times the bitrate in a little over an hour... using the exact same settings off an external HDD.

Needless to say I'm kind of at a loss here.

Thanks,

Specs if necessary:
i7-8700k @ 3.7Ghz / Intel UHD 630 enabled for hardware rendering
24GB of RAM
Radeon RX 480