Debugging it is a BIG pain. Roblox doesn’t provide us much tooling to debug what causes the spikes.
I can highly recommend this for debugging your remotes: Packet Profiler - accurately measure remote packet bandwidth!
It’s unlikely that remotes are causing issues though (unless you’re sending loads of data every frame).
What was the biggest issue for our game was attributes. They’re nice and handy, and I used them to store some properties on each character. These properties were updated every frame. This is good, but the issue is that roblox automatically replicates those attribute changes to everyone. This causes massive bandwidth usage for replication.
I suggest thinking about any (server-sided) script that runs very often (every frame or so), and debugging them. It’s trial and error, so not fun…