The rate is when you just open Bitcoin Core is misleading because,
1. Bitcoin Core only connect to 2 nodes during startup
2. There's not enough historic data (how long to download/verify each block) to make good estimation
This isn't supposed to happen because the Qt window is supposed to be in a separate thread from the transaction verification thread, and there's only one verification thread only, unless the UI is sleep-waiting on the verification thread to finish (??) in which case it makes sense that it freezes because the verification thread is constantly waiting on disk I/O.
It also happens to me (on Bitcoin Core 0.21.0)