noor I can't explain it either, sounds like you're using a 56k modem to download it, your connection being metered and throttled? A lot of variables on a persons system that could be impacting their USB and transfer speed.
I've done generational installs from Pixel 3 to Pixel 8 and never had it take so long with combinations of stock and non stock cables as well as between different Android devices with and without GrapheneOS as well as desktop/laptop environments. Never taken so long. Niche experiences like this are guaranteed to not be the norm.