The CPU load increases with the combined load of the two CAN C and D buses. The additional CPU usage can be approximated by adding 0.182 percent / kbps for transmit messages and 0.1 percent / kbps for receive messages.
For example, if the total transmit rate is 150 kbps and the total receive rate is 100kbps across both buses, the CPU load is approximately: 150 * 0.182 + 100 * 0.1 = 37.3%.
The chart below shows the relationship between CPU load and CAN bus load for an application that has a baseline CPU usage of 6.5% (no messages on CAN C and D). As a point of reference, a CAN bus with a speed of 500k transmitting 8x 8 byte messages, each at a rate of 10ms has a total bit rate of approximately 102.4 kbps.