This is an absolute non sens... a new computer should be faster, not the contrary!
This would be true if you gave it the same specs (SSD). An SSD is way faster than any platter drive. Having platters, the drive has physical moving parts like a record with a needle that has to go across the record to read the data. If data is spread out throughout the drive, then the needle has to jump all over to get the information you need. A Fusion Drive is a "hybrid" drive. Basically, it's a platter drive that stores all your information and then also has a larger Cache then a normal hard drive. It uses this cache to store information that you use frequently so that information can be accessed quickly as it is something that is used frequently so figuring that is data that needs to be stored there. It's kind of like RAM, your computer can't access any data without it being in RAM, so in order for your computer to be able to run a program or anything like that, it has to first be loaded into RAM by your hard drive. The hard drive is way slower than RAM so your computer has to wait for the drive. If you have more RAM, there is more data that can stay loaded on your computer at the time that it is needed so you can in fact then have more programs running or larger programs running and keeping them up to speed giving it a faster speed, however, this is not going to make the hard drive access data any faster, it's going to just make it so you don't have to load it every time you reopen the program. This will hold true until you restart the computer at which time it is then going to have to load it again.
Upgrading your RAM will make the computer itself faster, but it is not going to make the data load any faster, this is totally controlled by the hard drive. When you purchased the computer, you were given the option to go with an SSD in place of the Hybrid Fusion Drive, this would make it "faster" than your old computer because everything is going to access data faster. Once you make the decision, unless you are comfortable taking the screen off the computer and getting into it, you're going to have to take it some where that they are comfortable doing that and they will charge you for it as well as a different drive. It's not a matter of the computer being slower than before, once you load it or if you do things that are processor intensive such as video editing or photo editing, you'll notice that the new computer is easily going to outrun the old computer. Especially with having additional RAM