Hello Leo,
the document you have linked for 3500 XL says:
3500-XL 8,000,000 pps 10.8 Gbps of switching fabric
in theory it should be able to deal with 5 Gbps of traffic, the question is that datasheet are written with marketing purposes and all numbers should be halfed because they count separately tx and rx traffic.
1488000 pps are required per direction for a GE port using 64 bytes frames (minimum size)
it gives us 2,68 GE ports full duplex with minimum size.
Also the fabric is built in such a way that user ports to uplink port is the "stronger" side but for example performances between user ports or between uplink ports are lower.
With 1,4 Gbps on user ports how much traffic is local (between user ports), probably only a small percentage.
So the question may be also where users are connected, if for example a server is connected on second GE port, performance on moving traffic between GE ports is less than what is declared on the "strong" direction and this could explain the observed behaviuor.
These devices are legacy and cause a lot of problems also at the management level their IOS including last one is affected by bugs that make them unmanageable from time to time
Hope to help
Giuseppe
