I built a mob grinder in the overworld from here: https://www.youtube.com/watch?v=Gg17wAr_IOI

The rates are said to be 17,500 drops per hour but I am getting much less. Mob spawning is really slow, even though mob spawns on the ground at night seem to be normal. I am playing on a vanilla 1.14.4 server hosted by one of my friends. There were ~5-6 people on.

The grinder works well in singleplayer, as the video shows it does. It slows down massively on the server which sucks because it took me a while to build.

I used a client to highlight mobs and items. In the pictures, green boxes are mobs, yellow boxes are items.