Z420 E5-2670 128GB(Registered ECC 16GBx8)

WDC WDS500G2B0A
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CrystalDiskMark 6.0.0 x64 (C) 2007-2017 hiyohiyo
Crystal Dew World : https://crystalmark.info/
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* MB/s = 1,000,000 bytes/s [SATA/600 = 600,000,000 bytes/s]
* KB = 1000 bytes, KiB = 1024 bytes

Sequential Read (Q= 32,T= 1) : 562.959 MB/s
Sequential Write (Q= 32,T= 1) : 529.517 MB/s
Random Read 4KiB (Q= 8,T= 8) : 388.712 MB/s [ 94900.4 IOPS]
Random Write 4KiB (Q= 8,T= 8) : 331.344 MB/s [ 80894.5 IOPS]
Random Read 4KiB (Q= 32,T= 1) : 373.072 MB/s [ 91082.0 IOPS]
Random Write 4KiB (Q= 32,T= 1) : 256.339 MB/s [ 62582.8 IOPS]
Random Read 4KiB (Q= 1,T= 1) : 31.348 MB/s [ 7653.3 IOPS]
Random Write 4KiB (Q= 1,T= 1) : 61.463 MB/s [ 15005.6 IOPS]

Test : 1024 MiB [C: 80.3% (373.5/465.0 GiB)] (x5) [Interval=5 sec]
Date : 2018/07/07 20:52:13
OS : Windows 7 Professional SP1 [6.1 Build 7601] (x64)

Z420は、SATA3に対応したコネクタは2つしかないので、SSDが繋がっている
SATAコネクタの場所をHP Performance Advisor等で確認してみては？
それと疑うなら、SATAケーブルの品質が悪く、転送速度が落ちてるとか。

ttps://support.hp.com/jp-ja/document/c03228152