UE5 multiplayer & GAS problems

What was the hardest multiplayer, GAS or performance problem you hit in Unreal in the last 90 days? 5 short questions, about 2 minutes. Only the first problem question is required.

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1 Your role (optional)
A sentence or two is plenty.
3 What did it affect? (optional)
4 How long did it take to deal with? (optional)
5 How did you solve it (or try to)? (pick all that apply)
UE version, what you tried, or anything we missed. If you're open to a 20-minute follow-up chat, leave an email. It's used only for that.