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This code plots a 1000 point black body radiation curve in 0.7 seconds. 

h = 6.62*10^-34
c = 2.99792458*10^8
k = 1.38064880*10^-23
T = 3000.
Isubν[ν_] := ((2 h ν^3)/c^2) 1/(E^((h ν)/(k T)) - 1)

Plot[Isubν[ν], {ν, 1*10^13, 1*10^15}, AxesOrigin -> Automatic, PlotPoints -> 1000,
     MaxRecursion -> 1]

However, using Quantities with units, the following code takes 19 seconds for just 200 points. What am I doing wrong?

h = Quantity[6.62*10^-34, "Joules"*"Seconds"]
c = Quantity[2.99792458*10^8, "Meters"/"Seconds"]
k = Quantity[1.38064880*10^-23, "Joules"/"Kelvins"]
T = Quantity[3000, "Kelvins"]

Isubν[ν_] := ((2 h (Quantity[ν, "Hertz"])^3)/c^2) 1/(E^((h (Quantity[ν, "Hertz"]))/(k T)) - 1)

Plot[Isubν[ν], {ν, 1.0*10^13, 1.0*10^15}, AxesOrigin -> {1.*10^13, 0.},
     PlotPoints -> 200, MaxRecursion -> 1]
Sektor
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BillT
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    Units are slow, I'm afraid. By the way, Planck's constant, the speed of light, and Boltzmann's constant are built in: Quantity[1, "PlanckConstant"], Quantity[1, "SpeedOfLight"] and Quantity[1, "BoltzmannConstant"]. See https://mathematica.stackexchange.com/questions/16446/are-mathematica-9-and-10-units-really-slow – Patrick Stevens Oct 28 '15 at 07:38
  • As noted, these ones are on the slow side, unfortunately. Consider reformulating so that you are doing computations with natural units, or nondimensional quantities. (QuantityMagnitude[] is particularly useful.) There is also the new PlanckRadiationLaw[] function. – J. M.'s missing motivation Oct 28 '15 at 07:44
  • Thanks for the info, Patrick. I wrote the code first using the built in constants, then took them out to see if they were the culprit. I submitted my question with "typed-in" constants, just for simplicity, but thanks anyhow. I have looked at the question you referenced for me. A lot to study there. Many thanks. Bill – BillT Oct 28 '15 at 07:58
  • J.M.: PlanckRadiationLaw[ ]: Way cool, didn't know about it. Thanks. – BillT Oct 28 '15 at 08:03

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