I have about 750,000 files and 38,000 folders (I compile a lot of stuff). Are you putting everything in just one folder or splitting the folders up? I'm on really old hardware with mechanical drives and 100Mbps LAN so my observations are going to be different than yours.
I started out with just one folder syncing everything in '/home/kindone', that takes 30 minutes just to scan every time I started syncthing. Syncing over the LAN would not saturate my network unless it was a large file. That took about five hours to get everything synced when you have 700,000+ 4 - 400KiB c/c++ files. (It seems to send about 4 - 5 files at a time, someone correct me if I'm wrong.)
I removed '/home/kindone' and creating separate folders in syncthing for things like '/home/kindone/build/linux-4.14', /home/kindone/build/wine', and '/home/kindone/Dropbox'. Created about 65 different folder for various things, added them to my other machine on the LAN (that was annoying). Once they started syncing my 100Mbps LAN was saturated. Starting syncthing now takes 5 minutes to scan everything.