20 seconds is probably a too long time to wait if you can get it done with a cable in 1 second? My interest in cable-free operation is for safety but I am not patient enough to wait 20 s for a file to be transferred and I would rather use the cable in that case or if wireless transfer is mandarory, opt for a smaller JPG to get it done faster.
Nikon do provide some options for faster wifi (D6 can use the WT-6 which provides faster, longer range wifi and for that they do provide software options (camera control pro 2, wireless transfer utility)). For the D850 you'd need to use WT-7 which is a big bulky thing (the D6 gets away with a small plug-on accessory because most of the electronics for networking is inside the camera body). Z7 and Z6 can use WTU but not camera control pro 2 (using built-in wifi, so you get transfer to but not control using a computer). WT-7 option is available for them as well, though I find it hard to believe many would buy it.
For the D850, probably those third-party software options are your best bet. If it works then all is well, no? A lot of people wouldn't voluntarily install Nikon software on their computers, anyway.
(I do use it.)
There is also a product called Camranger that you could look into.
I can see Nikon's point of view that for professional-level wifi they want to sell accessories and software while built-in wifi is with basic capabilities for social media etc. they provide built-in capabilities. But I too think they should step up and provide (1) better software support for built-in wifi and (2) faster and more reliable wifi capabilities built in.
