Allow me to bring a compromise:
With custom maps in the mappool being a thing, why not have a custom warmup as well? This way, the organizers have total control over what map is played and can make sure the map fits to certain standards.
As mentioned:
•A lot of time wasted being ready, specially from teams with awful download speed trying to download warmups
Organizers can decide the length of the warmup themselfs.
•People pick BIG files for warmups to waste even more time
Again, organizers can cap the filesize. Since the warmup would be included with the mappool, there wouldn't be any long download times by any of the teams.
•Client sometimes has issues loading files, restarting the client takes time too
Guess what, the warmup can be tested by staff before it gets included in the pool! Conveniently it's also one single map, so that shouldn't take too much time.
•Matches start so late after warmups that ends going over the designated schedule missing other teams warmups.
Refer to point 2
•Inappropriate warmups, some of those takes time to get why is inappropriate because of source, hitsounds, long mp3, bg, different language, etc.
Obviously won't be a problem anymore.
•Warmups are uploaded close to the match so people have even less time to DL > more delays
..
While it's sad to see creative and funny warmups by the cwc teams go, I think this should be at least a decent option to consider.