It seems that you are having issues with your SRX5308. May you be able to check if it is running on the latest firmware version? You may also reset it to factory default and check if it will now work properly.
I just updated the firmware to the latest yesterday and could not get the full 300mbps. It did help the speeds by about 10mbps. I would rather try the full reset another time if possible. The office is normally full of people at different times of the day. Is there a setting that could throttle speeds somehow?
Technically, yes there is a way to throttle the speeds. You may check the QOS settings if it is enabled. When you are trying to do the speed test on the SRX5308, how many devices are connected to it? Have you tried connecting only 1 PC? Please make sure that you are connected directly to the firewall router and not in any panel or switch when doing the speed test.
