Again, I have no problems with my two iOS devices running iOS 10.3.3 and one old device running iOS 9.3.5 (which can't be updated). All three devices are running the Fastmail app version 1.2.5-406 in Production mode.
In addition to my earlier suggestions (checking the app version/mode and rebooting the device), I suggest looking at your
App Store > Updates tab and verifying that you don't have updates in progress on updated apps (which is often the case after an iOS update). Anything which is using up bandwidth will make the Fastmail app run a bit slower.
Of course, your internet connection speed will affect how the Fastmail app and other heavy remote data apps respond. Have you tried your device in a location with a fast WLAN (WiFi) connection?
- Using LTE connections (with two different cellular systems on two devices) I have download speeds of over 5 Mbps, and it's an order of magnitude faster at my house using WiFi. That's using the free OOKLA speed test app. When I send a test message to myself from my desk PC, my iPad and iPhone normally ding announcing the new message within a couple of seconds.
- I often use much slower connections and of course very slow connections slow down the Fastmail app. It's also possible that your internet provider or cellular carrier has a network connectivity issue with the Fastmail servers, but I find this to be a very rare situation which I can't remember seeing in several years.
- Fastmail does have occasional uptime issues, mainly due to their network provider. So be sure to check this link if you have trouble:
https://www.fastmailstatus.com/
- If you device is on a fast network using the current version 1.2.5-406 and it still hangs for a long time often, I suggest you file a Fastmail support ticket.
Bill