Sometimes its is really a bad experience when you are playing in a server while downloading assets when you move around, you get so much lag, high ping etc. So it will be nice to get to download all assets before entering the server to avoid lag , high ping and bug.
We need like a option to toggle in fivem setting to choose between download all server assets at start or download while in game, so people will select what they prefer.
This topic has been discussed at length prior; in fact, even I have a post from January 2021 with some CFX staff responses, going into the logic and possible implementations. Nothing was ever finalized, though, however I do still hope it gets looked at again 
The post I mentioned, for reference:
It is crazy how they ignore the most obvious thing. Look at the other MP, you just have to download the assets at the launch and you are all good to enjoy.
Not every one have good internet or PC to download things in game while playing, in my case as soon as a download start in game even some kbs i get a tiny lag so it ruin my experience, and when it download some huge assets my ping goes up and i get kicked by the server anti-high ping so impossible to play.
It will be cool to get the ability to download all assets before joining the server so we enjoy a smooth experience.
If you read through the linked thread, you can see it was far from ignored with a pretty detailed explanation of why it wasnāt prioritized along with some ideas of how it would work which is more complicated then just saying ājust download the assetsā.
If you say so, after all i donāt know the work behind but it will be really useful and helpful if they figure something out