We have same issue with R77.30, we created support request but they couldn't resolve it.
We have 200M/200M Internet connection, users speedtest results are between 1-10 Mbps and they also have slow web surf performance.
Firstly I added a rule on Firewall policy for my ip as source, destination is any and service is any then I added a rule on app & url filter policy for my ip as source, destination is Internet and service is any and also added exception rule for https inspection then I tested, speedtest results were between 50-100 Mbps. 50-100 Mbps was normal for me because 100-150 Mbps was in use by other users.
I belive there is something wrong with Identity Awareness or App Control & URL Filtering or HTTPS Inspection.