It is slower but not nearly as slow as you suggest, I had to run 6000 (not 60) to get numbers to compare.
In[112]:= AbsoluteTiming[
Table[Quantity[i, "Seconds"]/Quantity[1, "Seconds"], {i, 6000}];]
AbsoluteTiming[Table[i/1, {i, 6000}];]
Out[112]= {0.0190010, Null}
Out[113]= {0.0010000, Null}
