Hi,
I can cleary understand that the there is fragmentation for the UDP traffic which most probably is causing the latency. Reducing the MSS value will only avoid the fragmentation for TCP traffic and will not be application for UDP traffic.
During the time of the issue please provide me with the output of "get session frag" and also packet captures on end to end device along with snoop on the FW with debug flow basic .
As we are generating 1 MBPS of traffic which is either fragmented on the device or being recieved as the fragmented traffic is might be choking the fragmentation queues on the device hence causing the packet drops.
You can try to reduce the payload of UDP segment from application end or you can try to configure Path MTU to avoid fragmentation:
# https://kb.juniper.net/InfoCenter/index?page=content&id=kb7049&actp=search
This is all we can do on the firewall to improve performance.
Regards,
Rishi