usb 3.0 thumb drive speed limit

[Marat N.Afanasyev]

I wonder is there a speed limit on usb 3.0? I've bought

ugen0.4: <SanDisk> at usbus0
umass2 on uhub7
umass2: <SanDisk Extreme, class 0/0, rev 2.10/0.10, addr 4> on usbus0
da2 at umass-sim2 bus 2 scbus9 target 0 lun 0
da2: <SanDisk Extreme 0001> Removable Direct Access SPC-4 SCSI device
da2: Serial Number AA010808161609220143
da2: 40.000MB/s transfers
da2: 59840MB (122552320 512 byte sectors)
da2: quirks=0x2<NO_6_BYTE>

that claims 'Up to 245 MBytes/sec read speed'

and dd shows:

% dd if=/dev/da2 of=/dev/null bs=1m count=1000
1000+0 records in
1000+0 records out
1048576000 bytes transferred in 25.688997 secs (40818098 bytes/sec)

why we have such a limit?

-- 
SY, Marat

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