It's not just for SSD users, it's for everyone:

Our testing shows that for the small percentage of players still using mechanical hard disk drives, mission loading times have only increased by a few seconds in the worst cases.

Only a few seconds difference?

Further good news: the change in the file size will result in minimal changes to load times - seconds at most. “Wait a minute,” I hear you ask - “didn’t you just tell us all that you duplicate data because the loading times on HDDs could be 10 times worse?”. I am pleased to say that our worst case projections did not come to pass. These loading time projections were based on industry data - comparing the loading times between SSD and HDD users where data duplication was and was not used. In the worst cases, a 5x difference was reported between instances that used duplication and those that did not. We were being very conservative and doubled that projection again to account for unknown unknowns.

Now things are different. We have real measurements specific to our game instead of industry data. We now know that the true number of players actively playing HD2 on a mechanical HDD was around 11% during the last week (seems our estimates were not so bad after all). We now know that, contrary to most games, the majority of the loading time in HELLDIVERS 2 is due to level-generation rather than asset loading. This level generation happens in parallel with loading assets from the disk and so is the main determining factor of the loading time. We now know that this is true even for users with mechanical HDDs.

Good info, thank you.