To give you a reply, cyber, I'll say that there are changes to the code that can affect the way server does things, without need of having to use a new client. In that sense, those changes would be transparent to you and anyone else playing the game.
One of the things that you surely recall is database changes. They just need a server restart to be there. NPC appearance, for example (their uniforms/armours/etc) can be changed that way, without any need of a new client release.
Just as that example, there are many other things that can be "tweaked" without need of you to change your client. Let's assume I'm the server and you're the client. Imagine you give me a number that I've to return to you factorized. I've many ways to do this, but you simply give me a number, and expect the factors back. The way I obtain those factors is completely transparent for you. So, I could change how that is done, with the lack of fortune to have a less efficient way to obtain the factors, aka you'd observe a delay in my reply vs the previous implementation.
This just to say that some changes can be done serverside transparent to the user, and that it can be that changed/newly implemented functionalities aren't yet optimized enough, and cause the server to be on stress, thus giving you (and others playing) that feeling of lag.
Once upon a time there was a way to obtain statistics from the server, so you could actually check its memory and cpu usage. As far as I know, this is no longer possible, thus what I'm telling you here is mere speculation, though it could explain what you experienced.
Keep in mind that, since you say others playing the game also had that same feeling, it's not a problem on your end, so you shouldn't worry too much about it.
Hope this is what you were asking, and that it can give you some answer.
