Three different numbers get lumped together as "internet speed." Only one of them is fixed by the laws of physics — and it's usually the one making the call feel laggy.
When a video call stutters or a game feels delayed, the instinctive fix is to buy a bigger internet plan. Sometimes that helps. Often it does nothing at all, because "slow" can mean three completely different things: how long a single piece of data takes to make the round trip (latency), how much data the connection could theoretically carry per second (bandwidth), and how much data it is actually moving right now (throughput). Bandwidth is the one thing an ISP can sell you more of. Latency, for a given destination, mostly isn't for sale at any price — and it turns out to be the thing that governs whether a connection feels responsive.