When the total amount of traffic aggregated from the source ports exceeds the physical limitations of the destination port, it results in some dropped packets on the destination port. This does not cause any switch performance degregation or disruption or traffic flow on the source ports. The only affected port is the destination port, and it drops packets on a First In-First Out (FIFO) basis once the egress buffer limit is exceeded.
Whenever you create SPAN sessions with many ports for sources, there is a performance impact as now the switch carries twice as much traffic. Each linecard replicates the traffic at ingress, whenever you SPAN a port all ingress traffic is doubled when it hits the fabric. If you capture traffic from a lot of busy ports on a linecard, it can fill up the fabric connection, for example, the 6548 linecards which has 8 gigabit fabric connection.