Slow random read benchmark performance, but write OK

[ocoro02]

Hi - device is a TS-459 Pro II, firmware 4.2.6 build 20171208. Disks are 3x WD Red 3TB arranged RAID 5, no errors or problems showing up, SMART test results good.

I haven't run benchmarks for a while but as I'm thinking of getting another NAS I thought I'd check out how this one is doing, so I'm running Crystaldiskmark from my Windows PC to the Qnap, standard tests (5x 1GiB files per test).
Rather strangely in Crystaldiskmark I'm seeing the read speeds from the random 4KiB chunk tests (Q8T8, Q32T1) a lot slower than the write speeds - like 1/4 of the write speeds! (7MB/s vs 29MB/s). Anyone any idea how this could be? The Seq and Q1T1 look as I'd expect them to be. I've repeated the test a few times with the same results. Network is configured OK (I've tried both single ethernet and port trunking - balance-alb - not that it'll make any difference) but as the Seq test is fine it almost certainly isn't that. All I can think is some sort of fragmentation (ext4 filesystems so probably not) or read caching oddity?

-----------------------------------------------------------------------
CrystalDiskMark 6.0.0 x64 (C) 2007-2017 hiyohiyo
Crystal Dew World : https://crystalmark.info/
-----------------------------------------------------------------------
* 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) : 117.786 MB/s
Sequential Write (Q= 32,T= 1) : 108.303 MB/s
Random Read 4KiB (Q= 8,T= 8 ) : 7.047 MB/s [ 1720.5 IOPS]
Random Write 4KiB (Q= 8,T= 8 ) : 29.782 MB/s [ 7271.0 IOPS]
Random Read 4KiB (Q= 32,T= 1) : 7.131 MB/s [ 1741.0 IOPS]
Random Write 4KiB (Q= 32,T= 1) : 29.466 MB/s [ 7193.8 IOPS]
Random Read 4KiB (Q= 1,T= 1) : 8.716 MB/s [ 2127.9 IOPS]
Random Write 4KiB (Q= 1,T= 1) : 1.508 MB/s [ 368.2 IOPS]

Test : 1024 MiB [] (x5) [Interval=5 sec]
Date : 2018/02/23 20:16:06
OS : Windows 10 [10.0 Build 16299] (x64)