This problem there are more than 5 Years !!! And apparently bitdefender company don't want to resolve this problem because of don't care to us
so This is solution to solve the problem [Please do step by step] :
Important : protection still there are only for this 3 browser (Mozilla FireFox , Google Chrome , Safari)
1- Open "Folder Option" and go to "View" and Uncheck "Hide extensions for knows file type" and click ok
2- right click in free space and create new text with "New>Text Document" (pay attention : Don't open this file) , now rename "New Text Document.txt" to "bdtl.dll" and Click yes to change extensions
3- Go To "Safe Mode"
4- copy bdtl.dll file and go to "C:\Program Files\Bitdefender\Bitdefender 2016" and paste here and replace !
if you have bitdefender 2015 go to "C:\Program Files\Bitdefender\Bitdefender 2015" and do same step
5- right click on bdtl.dll and go to Properties and now check "Read Only" and click to apply and now go to security Tap and click on Edit and we have 4 or more options ( security , Administrator , User , All application , ... ) and click on "All application" and check Deny on "Full Control" Option (check all deny options) also for other option like security , Administrator , User and ... (check deny "Full Control") !!! in finally click on ok !
like this pic : http://www.axgig.com/images/09976681233029192431.jpg
6- Restart System
7- go to :
http://www.bitdefender.com/solutions/trafficlight.html
In the end , download and setup plugins on your browser !
Now we have resolved Bitdefender (resolved "the problem of drop-Speed Internet") without turn off any protection 
note : After doing this solution please ignore "Restart" Pm in Events !!!
Have a Good Day