When I change chunk (move from one chunk to another), there's always a freeze (edit: as said by Jay Eff in the comments, it starts a while after first loading the world for me too). See
[media]for illustration.
Happens since 18w43a.
Also, it doesn't happen on Minecraft 1.12.2 or 1.13.2.
Tested in 18w43c :
[media][media]Launcher profile :
"1af83bf96f3edd2089a6f2e172a92549": {
"type": "latest-snapshot",
"lastUsed": "2018-10-26T09:21:54.647Z",
"javaArgs": "-Xmx4G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M"
}(Personnaly confirmed this bug in 18w43a and 18w43b (and not present in 1.12.2) with default unmodified profile = without custom java args)
Can confirm, 18w43c. If I'm walking and looking forward, the noticeable framerate stutter happens when I see new scenery appear in the distance (which is what happens when I'm crossing chunk borders).
It's a little strange, because there's no stutter at all while a new world is drawing when first spawning into it, but the world is first drawn in sort of a 'splatter' effect instead of all of the visible world at once.
I don't know anything about Minecraft code, but maybe the chunk-crossing stutter is happening because the server is passing to the client all of the new chunk data all at once and then the client is drawing it all at once, when maybe it should send all of that new chunk data one-chunk-at-a-time on every other tick. Or, if the client-server communication is on its own thread, then send all of the data at once but draw it in on the client side with the same dappled method as what happens when first spawning into the world, instead of drawing all of the new data at once immediately after it's received from the server.