Well, a 50 megabit Internet connection doesn't mean that a mail server can keep up with that...
Is it -- until emails start to appear at all? Or the time between the headers in the message list and message body snippets?
Do you have "SSL hardening" enabled in app settings under Networking? Can you try turning it off, then rebooting the phone?
I have noticed slowness of message snippets appearing recently. At the time I blamed the connection I had (tablet tethered to iPhone, with a weak 3g signal). The slowness was not just delay to the first, they came in rather slowly one at a time. Headers were about as fast as usual.
OK my servers, connection, everything is possibly different from the OP, but thought the info would help.
On my own home WiFi I noticed another effect only once. Where a failed sync results in a red time next to the account name, I recently saw messages sync after switching on my tablet, and 1or 2 new messages were downloaded. The time next to the account name appeared, this time in black, something like 02:19 (probably the last sync before I turned off ). The time of the messages was later than 02:19.
Calling for a sync all once more did nothing except move the rotating doughnut through each account in turn (no new messages, time still 02:19), then on a further manual sync the time updated to the real time 21:xx, and some more messages appeared.
This was a one off issue. Haven't seen it since.