If you read the section on the DAC in the datasheet you would surely have seen this:
45.6.1 Digital-to-Analog Conversion
The DACC uses the master clock (MCK) divided by two to perform conversions. This clock is
named DACC Clock. Once a conversion starts the DACC takes 25 clock periods to provide the
analog result on the selected analog output.
Since the MCLK is 84MHz that means DACC clock is 42MHz and conversions take
about 600ns, so, but remember if driving both channels there is only one converter
shared between them.
There is also overhead in the call to analogWrite().
170Hz isn't going to be a problem, the DAC is designed for audio rates.
The problems you may not have appreciated is that the output is not rail-to-rail, only
spans the middle 2/3rd of the supply range. Also the 12 bits is not realised in practice,
you may be better off with a separate DAC.