Three factors can limit the speed of photosynthesis:
Carbon dioxide concentration
1. Light intensity
Plants need light for photosynthesis. Without enough light, a plant cannot photosynthesise quickly. Increasing the light intensity will increase the speed of photosynthesis.
2. Carbon dioxide concentration
Carbon dioxide is a reactant needed for photosynthesis. If a plant cannot get enough carbon dioxide photosynthesis will be limited. Because the concentration of carbon dioxide is increasing in the atmosphere, researchers have found that trees are growing more quickly.
Temperature is also a limiting factor. If the temperature is too cold or too hot, then plants cannot photosynthesise quickly.
The availability of water is also a limiting factor.
Any one of these factors could limit the rate of photosynthesis.