FWIW, this sounds like a handshake issue at the server end to me....I too have seen this reload to the logon page recently with one of my financial institutions. For now, it's never more than a single event, but my internet connection is somewhat faster than yours....
I suspect that you're right. Because of my data transfer rate (in the low KB/s range) my connections have often failed on initial contact because they time out (and then require reload).
Add to that that all intermediate sites which any given URL might need to access in setting itself up must also undergo a TLS handshake
en passant. (I monitor the activity bar at the bottom of Firefox's window. Anyone with faster connection speed than I probably wouldn't even see what's in process.)
It's wholly unpredictable. I've just gotten used (almost inured) to it.
