Network Latency when using Vlan

[sbrisson]

I have a stack of 4 X 5698TFD-PWR. I have multiple Vlan assign to this switch and everything works well (speed is fine, No network Issus. ) I needed to add another cluster because I cannot cascade a 5th switch into the current one(max of 8 per stack and the 5698 count has 2 ) I have a stack of 2 x 5952GTS-PWR. Between I have 2 fiber connected at 40gig both ports in trunk mode and in MLT with all the required Vlan. When a computer is connected into the 5952GTS stack on Vlan 1 everything is fine (speed and network). When I tag a vlan on the 5952GTS the network speed is reduce by about 60% when I transfer a file from a laptop connected to 1 port into the 5952GTS to a server connected into the 5698TFD. ( all is fine when I perform the same thing using Vlan 1) No ping drop, network access is also fine but a big Issus with the speed. Port is configure for 1 gig, Advertise at 1 gig but the transfer speed between a computer into the 5952GTS and the server connected into the 5698TFD is really bad. Any idea's what may cause this latency !?