Why your internet slows under load: bufferbloat and the router queue

Why does it stutter when speed is fine?

Most people measure slowness as speed: how many megabits am I downloading? But what decides whether a video call or a game feels smooth is not speed, it is latency. How long does a packet take to travel there and back? That time is low when idle; if it spikes when the line fills up, the experience falls apart.

Your result card shows this as two numbers: your response time when idle, and your response time under load. The wider the gap, the bigger the problem. The line "your response time went from {ping} ms to {loaded} ms" is exactly this.

What exactly is bufferbloat?

Your router has a queue (a buffer) for packets waiting to be sent. When the line is idle the queue is short and packets pass without waiting. When a download or upload starts and fills the line, the queue swells: every new packet waits behind dozens ahead of it. That waiting is what comes back to you as latency.

That is where the name comes from: buffer + bloat. A big buffer sounds good, but an oversized one quietly inflates latency.

In everyday terms: you are on a video call, someone at home starts uploading a large file, and your audio starts breaking up. Your speed is still high; what stutters is latency.

What suffers most?

• Video calls (Zoom, Meet, WhatsApp): audio and video depend on instant response, so they break first.
• Online gaming: as your latency rises relative to opponents, characters teleport and shots miss.
• Live streaming and screen sharing: the queue swells during upload and the stream freezes.
• Even ordinary browsing: opening a page while another download runs makes clicks feel sluggish.

Reducing bufferbloat on your router

1. In your router's settings look for QoS (Quality of Service) or SQM (Smart Queue Management). Some brands label it "gaming mode", "video mode" or "smart queue".
2. If SQM is available, turn it on and enter your line's real download/upload speed. SQM deliberately keeps the queue short so latency cannot spike.
3. If your router lacks this, you can add an SQM-capable router behind it (OpenWrt-based devices do this well).
4. The upload direction usually swells more. When you re-test, watch the upload separately.
5. After the change, run the test again: if idle and under-load response times have moved closer together, it worked.

Careful: if you enter a speed higher than your line actually delivers, SQM will not work. Run a wired test first to learn your real speed.

So would a faster plan fix it?

Usually not. Bufferbloat is a queue-management problem, not a capacity one. A line with bufferbloat at 100 Mbps can still have it at 1000 Mbps, because the queue swells the moment you fill the line. Spending on a router that manages the queue helps far more than spending on raw speed.

Exactly how we measure latency under load (bufferbloat and RPM) is laid out in our methodology paper. Measurement methodology