Opening an email via the web can never be as fast as opening one via a local cache.
Also, taking more than 10 seconds to open an email is just insane on its own.
Old outlook took forever to launch, and would sometimes hang for no god damn reason, but at least it would somewhat instantly open an email.
New browser based, slop injected outlook opens moderately quickly, but still hangs for no god damn reason and can't open an email from notification in less than 10 seconds.
If not for vendor lock in and paid(?) shills, no one would ever use that garbage.