There have been three types of Nan Flash technology through the USB/SSD industry.
These are SLC, MLC and TLC (with QLC to be introduced Soon). SLC Nand employ faster speeds and longer life but at a significantly higher cost while on the contrary TLC is slower has slower performance and shorter life(when compared to SLC) but is less costly.
PNY USB Drives and SSD drives use TLC Nand memory.
Thanks to developments in Nand Flash technology TLC Nand is now able to implement a SLC cache to improve performance and reliability. SLC cache is not true SLC Nand but is employed by reserving some space in the TLC Nand storage for 1 bit per level saving of data.
TLC inherently has lower Read/write speeds and SLC Cache is the approach to boost these read write i/o operations. the concern with the SLC cache implementation is that once the Cache is filled and the data write-in outstrips the load of the SLC cach the performance drops as it will now have to write directly to the TLC Nand while the SLC Cache is cleared then the cache is able to be used again. This is not the case with non SLC Cache Drives. Please refer to the example image below.
Please also keep in mind that production of USB drives vary greatly per production assembly and do not use the same controller/Nand combination every time, because of this the SLC cache will vary on USB's even when the model is the same one.