After I enabled SSL Forward Proxy on SRX1500, the client traffic would decrease 90%. I used to download from the dropbox with 1Mbps, right now it couldn't reach 100kbps.Is that a normal behavior ?
Can you paste your configuration and preferably trace-options?
Also mention if there are any other changes being made apart from ssl proxy.
Enabling SSL forward proxy does take a toll on the device and the performance/throughput would decrease but in your case it is a huge hit.
Please share if all the traffic is forwarded through the proxy?
Also I would suggest opening a JTAC case to investigate this further to see if there is any software issue/performance limitation.