Thick disks aren't really faster then thin anymore (even much slower in 4K writes, especially with flash) also I'd recommend use the correct virtual nic (VMXNET3) and virtual controller (PVSCSI).
Passthrough will give you best performance (throughput and lower CPU) and best power savings as the disks can spindown.
Best way of doing XPENology on ESXi 6:
1) Boot ESXi from other controller (SATA/PCIe/USB) if using passthrough. SATA/PCIe recommended as you can use that as datastore for other VM's and the XPENology boot vmdk.
2) Passthrough extra SATA controller or the onboard (requires map file edit, nothing else can be on that controller) if using passthrough.
3) Modify image to block IDE and set NIC mac.
4) Modify VM to use VMXNET, PVSCSI (when not using passthrough) and set MAC.
How to do step 2 for onboard Intel controller:
1) Login to ssh console and run 'esxcli storage core adapter list'
2) In the description between parenthesis you'll see your pci id, something like: (0000:00:11.4) Intel Corporation Wellsburg AHCI Controller
3) Run lspci -n |grep 0000:00:11.4
4) After the last colon you'll see your pid, something like 0000:00:11.4 Class 0106: 8086:8d62 [vmhba0]
5) Now put '8086 [PID] d3d0 false' at the bottom of /etc/vmware/passthru.map, my full file:
# passthrough attributes for devices
# file format: vendor-id device-id resetMethod fptShareable
# vendor/device id: xxxx (in hex) (ffff can be used for wildchar match)
# reset methods: flr, d3d0, link, bridge, default
# fptShareable: true/default, false
# Intel 82598 10Gig cards can be reset with d3d0
8086 10b6 d3d0 default
8086 10c6 d3d0 default
8086 10c7 d3d0 default
8086 10c8 d3d0 default
8086 10dd d3d0 default
# Broadcom 57710/57711/57712 10Gig cards are not shareable
14e4 164e default false
14e4 164f default false
14e4 1650 default false
14e4 1662 link false
# Qlogic 8Gb FC card can not be shared
1077 2532 default false
# LSILogic 1068 based SAS controllers
1000 0056 d3d0 default
1000 0058 d3d0 default
# NVIDIA
10de ffff bridge false
# Intel Wellsburg AHCI
8086 8d62 d3d0 false
8086 8d02 d3d0 false
6) Reboot, put device in passthrough, reboot and attach to VM.
For step 3 see 'Modify the bootimage' here: https://idmedia.no/projects/xpenology/installing-or-upgrading-to-xpenology-5-2-or-later/
In your case I'd use an extra SATA controller (LSI based) as you don't have an Intel controller.
