While the bulk of the statement that “drones cause lag” is true in concept… in practice, it won’t cause lag unless you’re over-straining the node.
Lag, in a nutshell, is when the server can’t perform the calculations it needs to perform in the time that it has to perform them.
Imagine, for example, counting to 10 out loud every time someone pushes a button
Normal day to day stuff, there’s just one dude pushing the button every now and then - you’ll be able to say the numbers nice and slow. But start adding dudes. Eventually you’ll reach a point where say 5 dudes are pushing the button one after the other… you of course can only speak so fast, and so the dudes start to observe “lag” in how quickly you handle their input.
But it’s not an issue in normal day to day stuff in Eve, because the nodes aren’t all on the same hardware. They’re spread out, so that we can do things like have mining drones. A fleet of orcas, for example, will have no impact on the largescale fight happening elsewhere so long as it’s on a different node.