To clarify the newer Intel and M series use raw Flash chips not a true SSD
FYI: SSD has the same flash chips but also a controller chip and sometimes even RAM all fitted onto a small logic board which is called a SSD as the interface either complies to a standard like SATA or M.2 or could be custom which is the case in some Mac systems
Sadly the flash chips in this series are soldered to the logic board and not replaceable! Here's an image of your logic board and the Yellow outlined chips are the Flash chips on this side with two more on the other side (a total of four pad sets and depending on your purchase you can have 2 or 4 chips mounted (512GB, 1, 2 or 4TB)
But I'm not thinking your issue is the size, more likely you have too full a drive so the system needs to work harder to find enough space and/or you have to much loaded up in RAM and with limited RAM the system leverages Virtual RAM which in truth is stalling storage space (flash) to extend the RAM space. Again forcing the system to work much harder to meet your demands.
So I would first make a full backup of your system using TimeMachine to an external drive. I would then trim down what your drive has. a 512GB drive should have about 1/4 to 1/3 left unused so the system has breathing room. More if your systems RAM is limited to 16GB and you are running heavy apps or work with large data sets or have a lot open at once.
While I started off focusing on storage RAM is also something you need to look at as well. I often find people just assume you can run tons of apps or tabs in the 100's! Each app and/or tab uses RAM space and as you noted it takes a long time to start up which is a sign you have stuff being preloaded (reopening what you had open from the last time you booted up or logged in).
Think of it this way... You have a school bus which is parked at the school you arrive to the school bus to drive it off with the students. As you arrive before they are dismissed you can't take off you need to wait for them to get onto the bus! Once they have all piled in off you go. If you where at the bus yard you could just have gotten in and taken off no need to wait until you picked the kids up!
So make sure you don't have anything preloaded or when you shutdown you don't check off the check mark to resume as that will force the system to load everything you had open before. Or at least understand that's your demand on the system.
So why is the system so slow running when I run performance tests? Simple! Your drive is too full! Flash (SSDs) need to do house cleaning to free up space more so when your drive is overly full.
It's like the sweepers cleaning up after to horses or elephants trying to keep up with them. The bigger the herd is makes the poor guy work harder trying to keep up. One guy couldn't possibly keep up with a heard of 200 beasts!
So the issue is not the system its more an issue of housekeeping yours!