The initial load of any route created in a previous version will be the longest due to the route being converted over to the new format. Once saved and going forward, after the initial long load time, the loading process will be much quicker.
In addition to the old route conversion, any assets that are loaded the first time out, are being cached to help speed up operations in-game. Later on the asset caching (precaching) operation will be nearly invisible.
I have a couple of very, very large routes, with one weighing in at nearly 450 MB. The route took about 2 minutes to load instead of 6 minutes on the first go around.
If you are using a regular hard disk, I recommend defragmenting the drive. That really does help with the performance and with data loading times.