What type of drive is in use: SSD, Hybrid, conventional? If the latter, you may be in for a long, if not terminal wait. Be sure that drivers are up to date.
VP runs on 64-bit systems, certainly, but lengthy, multi-effect or UHD work is known to have 32-bit limitations resulting in errors, often in export, that mirror your description. While working with some of those projects here on an eighth-gen I7, near steady 100% CPU utilization has been seen.
The load of your raw clips showed 
at the outset, tapering off to a steady 22% or so. During an export attempt it was about 32%.
Yet caching those nine-minute clips in both the bin and on the timeline took in excess of an hour before the UI was fully responsive. Adding effects necessitated re-caching, though response remained good. Various resolution exports failed.
Softball Dad wrote: Are you suggesting that VP does not have the capability of editing 18 minutes of UHD video?
Empirically, from a practical standpoint, yes, though the sequence of your short, edited test project cached in minutes and played fine.
The most recent comment from a company rep stated that VP 64-bit is under testing. This isn't a one-trick pony: NCH has been around for more than a quarter-century. Many of their programs are reportedly slated for conversion. No easy chore.
In the interim, test pre-conversion at 2K (or 1080), possibly at 30fps. Your reticence notwithstanding, that, and divvying the project into discrete exports are alternatives. No guarantees. Downsampling your two files to 2K with another app took 10-15 minutes.
Sorry, but can't offer more. Report your concerns at https://tinyurl.com/y3mhbljb No rock stars here - mostly just users who don't work or speak for NCH.