If you are only using 11GB, then you must not have any personal data or third party apps installed. How much Free space do you have on the SSD? The only place where this is actually listed in macOS is within Disk Utility.....click on the "Data" volume in Disk Utility and the Free space will be listed on the right pane just beneath the storage bar graph. You need to make sure to always have at least 20GB+ of Free space. Ignore the "Available" value which macOS shows in multiple locations, but the "Available" value is very misleading.
Unless the SSD is failing, the decreased performance is more likely from some third party software you have installed, or a configuration issue, or an issue with the File system.
Run Disk Utility on the hidden Container (use the "View" option mentioned previously to see the hidden Container). Even if First Aid says everything is "Ok", click "Show Details" and scroll through the report to see if there are any unfixed errors listed. If there are any unfixed errors listed, then try running First Aid while booted into Internet Recovery mode, otherwise you will need to perform a clean install of macOS by first erasing the whole physical drive (Intel Macs only) before reinstalling macOS & restoring from a backup.
To look for possible software issues, run EtreCheck and post the report here so we can examine it for clues.
If you want to check the health of the SSD, then run DriveDx and post the complete DriveDx text report here using the "Additional Text" icon which looks like a piece of paper since the health of an SSD must be manually interpreted from the SSD's health information report. Unfortunately the Apple SSDs don't provide much health information, but it never hurts to check.
Also, make sure to disconnect all external devices in case one of them is causing a problem.
FYI, there are two 2013 models... an Early-2013 and a Late-2013 model and IIRC, they utilize different SSD connectors.
IIRC, a third party NVMe SSD will only be able to double the write speed of the original Apple OEM SSD for the Late-2013 model, but this will not really be noticeable during normal use although you may notice a faster boot time. For the Early-2013 model the SSD speeds are about the same as the original Apple OEM SSD since both are using SATA III based SSDs whose speeds are limited by the SATA III controller.