USG Flex 700 & GS1920-24HPv2 Throughput Issues

Hello all,

I currently have a USG Flex 700 connected to a GS1920-24HPv2 with a 4 Port LAG.

I currently have 5 vlans (1, 10,20,40,50) using the lag0 port on the USG and the T1 LAG on the switch. I have normal IP Addressing using 192.168.xx.1/24 on the USG and 192.168.xx.2/24 on the GS1920.

My issue is with a loss of throughput using the switch. If I connect directly to the USG, my speeds are steady at 560 mbps, but once I connect through the switch, my throughput falls significantly to around 140 mbps and also drops the longer the test runs.

I was thinking of attempting to use traditional inter-vlan routing (one vlan per port) to see if this corrects the speed loss, but it will take down my network to completely reconfigure (and I am not certain on the setup for this scenario such as tagged/untagged for the switchports as I used to be a Cisco guy and they are more straightforward), and I feel it should work properly with router-on-a-stick scenario like I am using.

Can anyone help me out to get the throughput issue corrected? I want to upgrade to gig internet, but it is not worth the cost right now with my throughput losing a majority of the speed through the switch to firewall connection (I have upgraded and the situation is very similar: 990 mbps from the firewall, and 140-180 mbps when connecting through the switch. I cannot afford a firewall with 28 ports to connect all of my nodes, and I have only had the USG Flex 700 & GS1920-24HPv2 for about 2 years. The USG runs great and handles the speed, it is only with the switch I end up bottlenecked.

Can this work with these 2 pieces of equipment, or do I have to get an L3 switch to end the throughput loss?

Thank you all in advance!