I've run into the same problem, my framerate is good (not excellent, but good) until I get to Whiterun, even with standard res textures, ENB turned off, and all settings turned to minimum. What's weird for me is that I've done this quest twice before, once on the Stormcloak side and once for the Imperials, with the same ENB setup and the High-Res Texture Pack enabled, with minimal slowdown. I don't know what suddenly changed on this playthrough. None of the mods I'm using should be that intensive.