These are some ways to solve this problem:
Use ISL trunks between core switches in the service-provider network. Although customer interfaces connected to edge switches must be IEEE 802.1Q trunks, we recommend using ISL trunks for connecting switches...
To have better and accurate result:
1- Use iperf of Jperf tool to test the bandwidth from your end to end node
2- Or connect 2 machine 1 at each end and start copying the file and observe the speed rate
This stack provides only half of the possible bandwidth between members and does not have redundant connections. Only the middle member has full stack bandwidth. The top and bottom members are operating at half the fully redundant stack bandwidth.