"Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight"[1]
If he were using this as a metaphor for the "speed of light" you'd think he'd be referring to the emails going incredibly fast, but what he's actually referring to is his "otherwise spectacular ... assistant" sometimes taking a very long time to respond to emails (because he's busy looking at lulz).
In this case I believe that the author is using "speed of photosynthesis" as a metaphor for being very slow, instead of using the more common simile "like watching grass grow".
I'm not sure if "speed of photosynthesis" is really correct, though - I find it hard to compare "rate of respiration" with "rate of growth" or "rate of email response" but there you go.
The last real science class I took was in high school, too. I don't actually have a problem with the metaphor, I was merely replying to the parent comment who said he was referring to the "speed of light".
The top level comment on this thread was saying he found the metaphor jarring - I don't personally find it jarring.
It can't be any worse than people who misuse the hammer and anvil metaphor.
Many say "You are the anvil, and I am the hammer" to imply superiority of the hammer bashing down on the poor anvil. However in reality, the hammer always breaks and the anvil always maintains.
Ah! I have another example like that. Sometimes people use the term 'quantum leap' to describe some huge change. As it turns out, a quantum leap is a very small change in the state of an electron, a very tiny event, nothing to write home about.
The point about a quantum leap is that it is discrete, rather than continuous, so, for example, a "quantum leap in understanding" implies a progress in understanding that cannot be broken down into a progression of small increments of understanding.
If he were using this as a metaphor for the "speed of light" you'd think he'd be referring to the emails going incredibly fast, but what he's actually referring to is his "otherwise spectacular ... assistant" sometimes taking a very long time to respond to emails (because he's busy looking at lulz).
In this case I believe that the author is using "speed of photosynthesis" as a metaphor for being very slow, instead of using the more common simile "like watching grass grow".
I'm not sure if "speed of photosynthesis" is really correct, though - I find it hard to compare "rate of respiration" with "rate of growth" or "rate of email response" but there you go.
He's certainly doesn't mean "speed of light".
[1]http://en.wikipedia.org/wiki/Photosynthesis