Are the iCloud+ email servers typically extremely slow?

I'm migrating my personal domain from GSuite and uploading my past emails is going at an excruciatingly slow speed of about one message per second. Is this typical? What sort of performance should I be expecting?

For more detail: I've become frustrated with GSuite because using it means your Google account cannot join family groups for things like YouTube and Google periodically break things like Google News meaning you cannot use those services while logged in to your GSuite account.

Now that iCloud+ supports using your own domain for emails, I decided to switch over.

I backed up my GSuite emails using Google Takeout, imported the mbox files into the macOS Mail app, and moved all the emails into my iCloud+ inbox.

The emails then started to sync but at an astoundingly slow speed. It's barely managing to upload a single message per second and occasionally gets stuck before erroring out and needing to resume. The whole process will take several days to import all my past emails.

Are the iCloud+ email servers typically this slow? I'm worried that I've made a poor choice in switching over.