[stumped]

If you still have the library accessible on the old pc, it may be faster not to use export ,Import. Aka backup, restore. Because those methods will pack and then unpack the database.
A calibre library is a single folder, so just copy the entire folder from old to new location. You must copy the entire folder, including any hidden file or metadata.db. a simple windows copy will preserve all the subfolder structure which calibre expects. Do it over lan or copy to a stick, walk it across the room and copy from stick to new pc.

For a lan copy , share the old folder so that everyone has read access and set up your pcs so they can see each other for file sharing, then give the copy.instrucion using the new pc to copy from remote machine to itself.

I have always used some variant of that to put calibre onto a new machine. Copying from old machine to cloud to new machine is another variant if you have enough cloud storage. I have never bothered with export import

On gigabit LAN, 15gb will be 10 mins or less,. With 100Mb ethernet links, not Gb then maybe an hour.
For gigabit LAN you need via it ports on. Both devices , and on any switch or router that is acting as middleman, and cat 5e or cat 6 cable.
If any one component is not up to spec the transfer will degrade gracefully to the slower speed.
For example I ran 20 metres of cheap, flat allegedly cat 6 cable from upstairs to downstairs, under carpets and across ceilings. It works,. But only at 100Mb. I can't face doing it all again with better cable !
Between devices on the same floor, with better cables, I get Gb connections