I have a strange issue I wanted to put out some feelers for.
I have a MS225*-24P with SFP+/10Gb uplink to WAN router (have tested both pfSense and SonicWall Supermassive) and several clients connected downstream.
1) Clients connected via copper GbE lose about 300-400Mbit of WAN throughput if connected to the MS225, speeds top out about 650Mbit/s. However, I have another 10Gb SFP+ downlink from the MS225 to a desktop client for testing - and full WAN speeds are achieved. Also, the same copper GbE connected directly to WAN router (bypassing MS220) achieve full near-gig speeds.
It seems there is some issue between the SFP+ interfaces and the the GbE interfaces on the switch?
I am having trouble understanding your configuration. The MS220-24P has Gigbibit SFP uplinks. It doesn't have 10Gbe SFP+ ports. Did you mean the MS225-24P instead?
Lets just assume the models mentioned aren't correct for the moment.
If I understand correctly, the issue is that data comes in via a 10Gbe port on the MS, and then goes out a Gbe port, and that you get around 650Mb/s of throughput.
My guess - the data is coming in the 10Gbe port at a rate faster than 1Gbe, causing a lot of data loss. You can confirm this by running Wireshark on the Gbe attach machine, and verifying that it is seeing lots of lost packets and there are lots of retransmissions.
10Gbe uplinks to desktop switches work well when you have a lot of users connected to a server. For example, you could have 10 users getting 650Mb/s each (as long as the server can supply it). If you had a Gbe uplink, the throughput would drop to a maximum of 100Mb/s per user. So a pretty good improvement.
If you have a device that is capable of high bandwidth streams (like SSD based iSCSI) and it doesn't have a way to back off well, then it can easily overrun the Gbe interface.