Wood is relatively soft and can be easily machined. If the on-site temperature is too high, however, this means that it tends towards burning, if you linger too long in one place with the router bit.
Elasticity, particularly for soft wood with a long grain, has a certain jamming effect on the router bit. This is then converted into additional frictional heat. It is particularly important to note the grain when it comes to solid wood, if you want to achieve good work results.