Hi,
There could be some many things to check, to start with:
1: What is the version of the vSRX and the type of virtual interface, e.g. VMXNET 3 etc.
2: What is the interface status speed etc, any errors, drop , pkt in/out in the 'show interface extensive'
3: show pfe statistics traffic https://www.juniper.net/documentation/en_US/junos/topics/reference/command-summary/show-pfe-statistics-traffic.html
4: Session details to see the WSF etc are negotiated properly.
5: Try to use only vSRX no other device , and perform your test to see if there is any improvement
6: CPU/memory status during the test 'show chassis routing/forwarding'
Thanks,
Vikas