Done even more tests. And all are the same. It takes little less then 10 seconds to open a ZIP file with 2880 jpg files inside.
14:05:11.530: Before I open ZIP = 0 ms
14:05:20.295: After file is opened = 8767 ms
14:05:20.311: After GetItems = 10 ms
14:05:20.530: After foreach loop = 217 ms
And while doing it, it freezes the entire application GUI, even if file is opened inside a BackgroundWorker thread.
I tried to open it like so:
using (ZipArchive zip = new ZipArchive(f.Path, ArchiveOpenMode.Open, ArchiveAccessMode.Read))
But no effect.