It's not lag, it's framerate (FPS) Frames-per-Second. Lag is when a connection, internet or otherwise, fails to transmit it's data fast enough, and causes hiccups or data loss entirely.
Yes, there is most certainly low FPS moments in Red Dead, and there is in almost all open-world games, but they are rare, and most certainly not game-breaking, unless you're an obsessive freak. Games operate at either 30 frames or 60 frame per second, depending. Consoles tend to use 30 FPS. The human eye cannot clearly see any difference with frames running faster than 30 a second. 60 is used sometimes, because it can allow for a more fluid experience depending on the kind of game and how the resources are handled.
In red dead, I imagine the game drops to roughly 20fps at times, and your eyes begin to see a "lag" in the frames. Mind you, it's temporary, either from so much textures or effects being processed by the CPU/GPU on the screen at one time, or a general resource clog. It's the most important aspect they had to work on for this video game, so rest assured you will not see anything game-breaking, trust me. Most of a game's development consists entirely of optimizing resources and trying to get the most of out them. Red Dead, is one of those resource-hog type of games, and they had to work for years optimizing and tweaking everything to get the most out of it, and I bet the frames don't go below 15fps at any given time. Which is quite professional.