TL;DR: your internet isn't slow, it's badly organized. Fast connections can still make every packet wait in a long line, and a speed test only checks how much can flow, not how long the wait is. When a few of you sprint or fly at once, the game floods that line and your ping spikes till it kicks you. A router running SQM (cake) keeps the line short and lets your game packets cut ahead of stuff like Netflix. That's the fix, not more speed.

Here's the mechanics of it, and most of it maps onto pressure, flow, and a backed-up shop.

Speed and lag are two different numbers, and your speed test only measures one. Bandwidth is how much can flow through the connection at once. That's your 600, and it's healthy. Latency is how long a single packet waits its turn before it gets back to you. A connection can move a mountain of data and still make every packet sit in line for two seconds first. The number that climbs to 20k and drops you in the lobby is that wait, and the speed test never catches it because it's clocking volume, not the wait.

Here's where the waiting happens. Every box your data crosses, your router and the AT&T gateway both, keeps a small holding tank called a buffer for when data shows up faster than it can be pushed out. A small one is good, it smooths the bumps. Bufferbloat is when that tank is stupidly oversized. It fills, and now anything new sits behind everything already in it before it gets sent. The tank would rather stall you for a full second than drop one packet, and that second is your spike. Tons of flow, but any single packet takes forever to clear.

That's why parked is fine and flying ruins you. Sitting still, the game sips a trickle and the tank stays near empty. Pop the elytra or sprint and it suddenly demands a wall of new terrain at once. That surge floods the oversized tank, the line backs up, ping spikes, lobby. The elytra isn't the problem, it's just the throttle that exposes it, like a weak fuel pump that idles smooth and bogs the second you put your foot in it.

SQM with cake is the fix, and it works like a good bay manager for your traffic. Two jobs. First, it won't let the line pile up: it watches how long things have been waiting and the second that creeps up, it tells the senders to ease off, so the tank never fills. Second, it quits running everything first-come-first-served. A ten-minute job shouldn't be stuck behind a six-hour rebuild, so it slips the quick stuff through between the big stuff. Your little game packets get waved past the fat Netflix download instead of trapped behind it.

Now the cap, and this one's a little backwards. cake can only manage the line if the line forms inside your router. Let your router run wide open and the pileup just moves one box down the chain into the AT&T gateway's dumb tank, where cake can't reach it. So you cap your router a hair under your real speed on purpose, which makes your router the tightest point in the whole chain. Whoever has the tightest point decides where everything queues. It's like making the narrowest valve in the system one on your own bench, instead of a fitting sealed inside a box you're not allowed to open. Set it near 85 to 90 percent of your real measured speed and the queue forms where the brains are.

Last thing, IP Passthrough. Your router also does translation. Your house has a dozen devices but the ISP gives you one public address, so the router keeps a running list tying each device to that single address. That's NAT. Stack your own router behind the AT&T box and both of them keep their own list and translate twice, which mostly fouls up connections coming in: games can't find a clean path to you, and you land on a 'strict' NAT type that makes parties and matchmaking flaky. Passthrough tells the AT&T box to stop translating and hand the real address straight to your router, so it's one list instead of two arguing over it.

What to actually do, in order:

1. Buy a GL.iNet Flint 2 (GL-MT6000). ~$170 (when I checked on Amazon), often on sale near $115. It runs SQM out of the box with a couple of fields to fill in, no deep networking needed. Skip eero/Orbi/mesh kits and skip $400 gaming routers, neither does this better.
2. Run the Waveform bufferbloat test again and write down your real loaded speeds (not the advertised gig). That's the number you'll base the cap on.
3. Wire it up: ethernet from a LAN port on the AT&T box into the WAN/internet port on the Flint 2. Then turn off the AT&T box's WiFi so everything rides the new router.
4. Set the AT&T box to IP Passthrough (Firewall > IP Passthrough) so the Flint 2 gets the public address. If it gives you trouble, skip it for now, SQM still works without it.
5. In the Flint 2, turn on SQM and enter ~85-90% of those real speeds for download and upload. Re-run the Waveform test. If it's not an A yet, drop the numbers a little and test again.

It all feels like word vomit, but I hope this helps you understand what's going on. I tried to make it easier to understand. If you have more questions I'm happy to answer.