I have done some measurements of high-intensity precision light sources myself for my internship, and always used a spectrometer.
The spectrometer (one from Avantes) can measure irradiance and radiance. In this case we had one with USB2.0 with a large and high precision spectrum, and were struggling to get 100 samples/second across (never looked into it, but I think it doesn't reach 50 updates/sec). We often used it at short (1ms) integration times ('capture'/'exposure' time) and was VERY noisy. Updates without averaging were also a bit noisy so we were using large oversampling rates for stable measurements (like 500x). This is all with the manufacturers software, I don't think there is an API to allow data extraction.
Moreover, at 'safe' irradiance levels (something that isn't going to directly blind people, like 1mW/cm2), we were measuring almost nothing at this short integration time. Only at potentially dangerous irradiance levels (like 10mW/cm2 or more) we were reaching full scales. If you want to do a DIY solution, you may encouter integration times as well, and may become a big issue if you're only catching low intensity light very quickly.
Furthermore a DIY solution is quite hard if want absolute precise values. Typical sensors have 'horrible' spectrums. Picking a sensor is fine, but if you want anything absolute you can't go without calibration from a known, precise light source. I am not saying it can't be done, but somehow those spectrometers and ready-made powermeters become to look quite cheap.
Also watch out you don't mess up radiometric (Watt) and photometric (lumen). Lumen takes sensitivity of human eyes into account and you can't convert it without knowing about the shape of the lightspectrum of your light source. If I quickly look at some of the sensors mentoined at Sparkfun, these are radiometric (they give you Watts).
If 30fps is too slow, what speed do you need?
Do you need the intensity at 1 spot, or is your goal to measure the power output?
Do you measure different light sources (even warm white or neutral white LED's can make a big difference)
I guess your biggest hit would be to search for a fast luminous flux, lux or lumen meter with USB2/serial support. You may need to modify your rig with a special set-up, integrating spheres or whatever to make it work for your situation.