For the first few hours it was coming in slower than my mail app.
Once it’d synced, it was coming through at the same speed or faster.
There are some annoying little things that have cropped up (mainly around notifications) on iOS, but I’m not sure if that’s because it’s still settling down (I’ve got a ton of old emails it’s syncing across, and I’ve added 4 accounts to it).
Ironically, the worst offender with Apple Mail was iCloud - It was SO slow to show that I’d read an email, and would often show the red “1” badge for ages afterwards.
I’ve heard good things about Outlook on iOS, but I like to try and use the same app on iOS and MacOS (and Outlook on Mac is woeful).
I also like that Spark integrates with Bear and Things.