When Router OS performs packet processing, are ICMP ping packets prioritized higher or lower or same as other packet traffic.

no priorities by default. you may use queues for prioritizing

Thanks - I agree from a forwarding perspective.

More specifically, is Router OS architecture single or multiple threaded with priority support. For example, if the OS gets ICMP packet, SNMP request and OSPF routing update at the same time, which one will it process first or is it on first come first basis. (aka FIFO)

as for packet receiving, it's FIFO. but take into account, that other packets can be received and sent to router facilities before some internal process is processing the packet. so it's multithreaded (but w/o internal 'priority' system)