Fan for MacBook Air?
It could help if you see kernel_task soaking CPU cycles.

From https://support.apple.com/en-us/HT203184


  Activity Monitor may show that a process named kernel_task is using a large percentage of your CPU, and during this time you may notice a lot of fan activity. This process helps manage temperature by making the CPU less available to processes that are using the CPU intensely. In other words, kernel_task responds to conditions that cause your CPU to become too hot. When the temperature decreases, kernel_task automatically reduces its activity. 


So if your case is more than 10°F / 5°C warmer than the ambient temperature and you are seeing high CPU/kernel task usage, cooling the bottom case or increasing airflow significantly might actually speed up your Air.