The polymerase is usually supplied in a glycerol solution to avoid freezing at -20C. I would guess this is the main viscosity factor of the PCR solution before amplification.

After amplification, DNA concentration most likely has a slight effect on viscosity, but this effect should be minimal as the concentrations aren't that high.

• Agreed
• The amount of DNA you add (~100ng) will not cause obvious viscosity
• Occasionally troubleshooting buffers will use substances like TMAC but in general it is the glycerol that imparts an element of viscosity (but not much in diluted working mix where Taq should account for no more than 10% final volume)

The PCR buffer composition could be the reason...... Try to read more about this.