we found that the default Fortinet DNS that are in FortiOS are rather slow.
So is SDNS if you use DNS filtering since it only uses US servers per default.
Setting system dns to e,g, Cloudflare or our ISPs DNS srvers together with setting the interface-select-method to sdwan and also setting this for fortiguard and have fortiguard anycast enabled and use aws as source improved that a lot.
"It is a mistake to think you can solve any major problems just with potatoes." - Douglas Adams