Mod content will load slower for a few reasons (some apply to Stock cars too):
#1 The car uses a high polygon 3D model
#2 The car uses a res textures e.g. 2048x2048 (~5mb each) and could use 1024x1024 (~1.33mb each) or even 512x512 (~350kb) for smaller components. So a mod that has a few 2048 textures on it weighs a lot! therefore loads are slower. FYI, after getting a car to make it to the spinner the first thing I do in optomise the textures to increase load times and MP on-track performance (As seen in the F1 2007 mod during beta testing)
#3 Most mods are not encrypted as the files within the .gcf's are and the encrypted files load faster, but make it hard to improve between version for modders
#4 The search that the game does for components starts in the .gcf's file structure then moves to the GameData et al folders
#5 Bad coding, missing 3D models, 2D textures, mis-used material naming, and many other little bits and pieces...