Users can connect to Minecraft server but time out during terrain download

I am trying to run a private Minecraft server for a bunch of friends and am using the dedicated server .jar file and running it through a .bat file with the following arguments:

java -Xmx3072M -Xms1024M -jar minecraft_server.1.12.2.jar nogui

My problem is, that I can join to my own server from my own machine, even through the external IP at any time, whereas my friends are unable to join the server once there is at least one person already playing. They keep receiving a Read Timed out-error during the “downloading terrain” phase while joining the server, or time out soon after they have successfully loaded and joined the server.

I have whitelisted Java in the Windows Firewall as well as opened the port 25565 in my router through both TCP and UDP.

canyouseeme.org also tells me that I am visible on port 25565.

Running a HTTP server on 80, or a Terraria server on 7777 works perfectly fine without any hitches.

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I have already tried:

• Allocating more RAM to the server (3GB)
• Opening every port to my computer through my router (Exposed Host)
• Disabling the router firewall to my computer
• Disabling the Windows Firewall completely
• Using a different port than 25565 for my server

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Some additional info that might be beneficial in solving the problem:

• CPU: Ryzen 5 1600 Six-Core Processor
• Motherboard: ASUS Prime B350-PLUS
• RAM: 16GB Corsair LPX DDR4-3000
• Router: FRITZ!Box 7390
• ISP: Deutsche Telekom

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Any help is greatly appreciated.