64-bit 11.86. Strung together six 1.5G 4K files (two-minutes each) that were created using H264@30fps in Matroska format. Added another 1.5 file as an overlay for good measure. Loading the files was relatively quick.
Twelve minutes, roughly 10 GB of content. Using a PC slightly less beefy than yours...
H265 4K export @45fps took 68.5 minutes, with average 10% GPU and up to 100% CPU. Exported file size: 153 MB.
H264 export completed in 3:27 with an average 95% GPU and 5% CPU. Exported file size: 2.88 GB.
Under OPTIONS | EDITING tab, at the bottom, DEFAULT was listed. You might try the other two choices. The sluggishness doesn't appear to be with Videopad.
Each install has a clean cache. The releases offered on the NCH site vary - sometimes they're 32, at others 64. The link above should fetch the latest 64-bit flavor.