Anyone know why others have gamebreaking lag on my create server[java]

i started up a server after having to get a new pc, but now the everyone else on the server but me has soo much lag it makes it unplayable, the onlything that has changed is my computer, which is more powerful than the laptop i was running the server off of before. were using the same modpack as before and im using the newest version of java. and before anyone asks, no i cant use ethernet, i would if i could but i cant.