What makes server loading slow?

Does anyone actually know what increases the amount of time it takes for players to join the server? Is it the amount of content they have to load or the amount of scripts?
For example, a darkrp server, usually loaded with tons of playermodels/cars/weapons that takes up at least 1 gb along with tons of scripts.

Its mostly due to a low or capped upload speed setted by the ISP or server provider, besides that no propper use of FastDL and workshop can really slow stuff down. I tend to stick to workshop to increase the speed since the Steam servers are all over the world. And other solutions I tryd to increase FastDL speed (Including Cloudflare) failed.

I never use workshop since it does not download everything. It might take a little while to setup fastdl, but it is worth it in the long run.