EDIT: The following pertains to the XPS 9530 from at least ten years ago, not to the new 2023 XPS 9530. Your system supports a maximum of 16 GB RAM. You should buy a matched pair of two x 8 GB DDR3L, up to 1600 MHz. Crucial is often the most compatible with Dell laptops. Here is Crucial RAM guaranteed to be compatible with your laptop. However, they don't currently show a 16 GB matched pair kit in stock, though you could check on Amazon or other Crucial retailers. If you want to save some money, you could try one of the lesser brands. I recently bought a relatively unknown brand, Kuesuny, through Amazon for much less than the price of a big brand name. I installed it in an XPS of similar vintage to yours. I'm happy with it. I ran memory tests for a couple of days to thoroughly test the RAM when I first installed it. It performs well and saved me money. You can upgrade your SSD all the way to 4 TB, if you want. Crucial's MX500 drives are good value, with almost the performance of higher-end Samsungs but at a more affordable price. Here are some Crucial drives. But, any major brand should be good. Given the laptop's age, then the thermal compound has dried out and cannot adequately transfer heat away from the processor, and there may be a lot of dust inside the laptop and maybe inside the fan and heat fins. So, your laptop is unable to adequately cool itself. That will cause the processor to throttle (slow down). You might consider an internal cleaning. Open it up to clean out dust. Disassemble the fan to clean out dust and fluff. I found a fair bit in two laptops I worked on recently. Remove the heat sink, remove the old thermal compound and apply new thermal compound before reinstalling the heat sink. Here's some information on what I did to an old XPS. While you're in there, replace the CMOS battery. If it isn't dead, it soon will be. BIOS settings and date/time can't be saved without a functioning CMOS battery. Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.
I ordered the XPS 15 9530. I want to upgrade the SSD to 1TB and RAM to 64 GB . Any recommendations on brands, types, speeds etc is appreciated. 
XPS 15 9530 Upgrades