I am not familiar with I2C, but I do notice that in your initial description you say the crystal is 8M, but in the code it is defined 20M...

The rest I do not know, but some thoughts...

Commenting the 2 I2C lines and recompile would give an indication of how much code is actually there (in I2C part)--and how that interface affects speed. The difference (between with no I2C and with I2C) should match pretty closely with datasheet transaction time for DAC chip?