Hi @rollinstech and welcome to the forum!
Yes importing large lists is slow due to DNS and other subscriber verification checks.
You could also try using the cli importer, not sure how it compares with regard to speed.
Another approach is to split csv files into separate ones and upload them separately. That helps with timeout or server upload limit issues.
There’s no way to import multiple files simultaneously that I know of.
If there’s no attribute data included (only addresses), then copy pasting into the simple upload page is easier than using csv, and probably faster.