Just to answer the prior question, your current C drive is the small SSD and your D drive is much larger but a standard HDD. This is a very typical configuration as SSDs are so much more expensive than the standard HDDs although they are (very) slowly coming down in price. There are many other ways to configure things, though.
TS3 runs mostly from RAM, so hard drive access becomes relevant on startup, saves, caching, world travel transitions, and when reading things like CAS and Build/Buy catalogs. Generally the game doesn't perform or lag that much differently on one over the other when it's not doing these specific things but you're kind of on the edge there with your 10 generation game and current video card so every little bit might be making a difference.
Many of us don't even have SSDs, my simming computer currently does not. So you certainly aren't alone in playing from an HDD entirely as you now are.
And yes, that company's flyer is making my eyes glaze over as well. You are in good hands here when it comes to picking out a new system. :)
