Lenovo E540: USB 3.0 HDD/ZFS: painful performance < 3 MB/s

Running recent CURRENT as buildworld of today on a Lenovo ThinkPad Edge
E540 with most recent Firmware 2.22 performs very bad having a USB 3.0
external HDD formatted with ZFS.

The casing is a FANTEC Alu U3e casing which claims to be SATA 6GB
ready, having external USB 3.0 and eSATA connectors, which shows n
FreeBSD CURRENT (FreeBSD 11.0-CURRENT #10 r292162: Sun Dec 13 13:27:19
CET 2015 amd64):

[...]
mptable_probe: MP Config Table has bad signature: \^A
Copyright (c) 1992-2015 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
        The Regents of the University of California. All rights
reserved. FreeBSD is a registered trademark of The FreeBSD Foundation.
FreeBSD 11.0-CURRENT #10 r292162: Sun Dec 13 13:27:19 CET 2015
    root_at_hermann:/usr/obj/usr/src/sys/SARIN amd64
FreeBSD clang version 3.7.0 (tags/RELEASE_370/final 246257) 20150906
VT(efifb): resolution 640x480
info: [drm] Initialized drm 1.1.0 20060810
CPU: Intel(R) Core(TM) i5-4200M CPU _at_ 2.50GHz (2494.27-MHz K8-class CPU)
  Origin="GenuineIntel"  Id=0x306c3  Family=0x6  Model=0x3c  Stepping=3
  Features=0xbfebfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,PBE>
  Features2=0x7fdafbbf<SSE3,PCLMULQDQ,DTES64,MON,DS_CPL,VMX,EST,TM2,SSSE3,SDBG,FMA,CX16,xTPR,PDCM,PCID,SSE4.1,SSE4.2,MOVBE,POPCNT,TSCDLT,AESNI,XSAVE,OSXSAVE,AVX,F16C,RDRAND>
  AMD Features=0x2c100800<SYSCALL,NX,Page1GB,RDTSCP,LM>
  AMD Features2=0x21<LAHF,ABM>
  Structured Extended
Features=0x27ab<FSGSBASE,TSCADJ,BMI1,AVX2,SMEP,BMI2,ERMS,INVPCID,NFPUSG>
XSAVE Features=0x1<XSAVEOPT> VT-x: PAT,HLT,MTF,PAUSE,EPT,UG,VPID
  TSC: P-state invariant, performance statistics
real memory  = 17179869184 (16384 MB)
avail memory = 16488468480 (15724 MB)
Event timer "LAPIC" quality 600
ACPI APIC Table: <LENOVO TP-J9   >


as

[...]
ugen0.2: <ASMedia> at usbus0
umass0: <ASMedia AS2115, class 0/0, rev 3.00/0.01, addr 2> on usbus0
da0 at umass-sim0 bus 0 scbus4 target 0 lun 0
da0: <ASMT 2115 0> Fixed Direct Access SPC-4 SCSI device
da0: Serial Number 00000000000000000000
da0: 400.000MB/s transfers
da0: 1907729MB (3907029168 512 byte sectors)
da0: quirks=0x2<NO_6_BYTE>

Writing to the raw, uninitialized device via

dd if=/dev/zero of=/dev/da0 bs=512M count=10G

gives ~ 30 MB/s.

The drive has been partioned with GPT, one partition covering the
whole drive with type freebsd-zfs which has been initialized with a
ZFS volume. 

What is wrong here?

Kind regards,

oh