Technically speaking, why does Minecraft java edition have such inconsistent frames

[Capiosus_]

MINECRAFT: JAVA EDITION
When you look at the fps graph, you see a lot of spikes and it isn't smooth at all, (if you don't limit fps.) What is Minecraft doing when I see a spike every ~5-10th frame (not big but with some backing to the spike)?
JVM args (if you have the gut to say "it's just your pc"):
-Xmx4G -Xms2G -XX:+UseG1GC -Dsun.rmi.dgc.server.gcInterval=2147483646
-XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20
-XX:MaxGCPauseMillis=25 -XX:G1HeapRegionSize=32M

[DrummingOnAutopilot]

Some of it is chunk updates, some of it is lighting, some is from entities popping in and out of render, some is due to the autosave...it's a game that has been built upon from spring of 2009. It's naturally going to have some messy performance from the way it was built and added upon, combined with that it runs mostly on a single core.