The BX500 series is a budget/economy model lacking a lot of features of the MX series. The BX series is not designed for sustained writing, but for reading data typical of a system used for web browsing, e-mail, etc. The BX500 is using TLC NAND which is much slower than older style NAND. In order to maximize the SATA throughput, it uses an SLC cache for writes, but when this buffer is filled you end up experiencing the much slower performance of the TLC NAND. All TLC NAND SSDs suffer from this problem and performance can be increased by the size of the SLC cache, the use of DRAM, etc. which increases the cost of the drive. TLC NAND is being used because it allows the manufacturers to increase the storage while also lowering prices. The MX500 series uses TLC NAND as well, but implements extra features so the performance does not drop quite as much for the sustained writes.
This review does a good job of explaining the BX500 series and why Crucial made such a drive. Page 2 of the review explains the sustained write performance which is relavent to your extracting the large RAR file.
FYI, I've writen this without seeing your screenshots since they haven't been approved by a moderator yet.
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