Assuming Warcraft III is already installed on a decent SSD, then long loading times are usually the result of poor scaling with some of the algorithms the developers choose.
The case I identified was with loading localised strings for abilities. An ability with 100,000 levels without any localised strings loads as good as instantly, but add localised strings for each level and it can take a significant amount of time to load. I suspect the scaling is worse than linear but did not take reliable measurements or resolve if it is due to the length of the localised string file, or the number of levels of the ability. This was given to the Reforged developers so I am unsure if fixes for that case made it out with their final patches or if they never had the resources to look into the issue.