Processed, as in routed? A small packet is routed as quickly as a large packet, though transmission time does vary depending upon the amount of data.
For example, a jumbo ethernet frame is routed at the same speed as a 100 byte frame, but the actual transmission time between the two will vary. And, of course, if the jumbo frame has to be fragmented due to a smaller MTU size on the transmitting interface, the packet will take longer to forward.
Also, after a packet is routed, it is delivered into a buffer for transmission, and if FIFO is being used, the packets are transmitted in the order they are received. QoS is a means of changing the order in which packets are queued to prioritize some packets over others.
I would recommend reading up on QoS, and cef (cisco express forwarding) for details.