Problem with HTTPS

Hello;

my name chaya, I use Mikrotik with FreeRADIUS. the problem is after user logging to system successful. they can open all website (http) but when they try to open https website such as yahoo mail, google mail or facebook it really slow to redirect to those pages sometime page can not open. what the problem? plz help me

Just check the MTU / MSS

M.

thank maximan, I am new in mikrotik. can you me more information about MTU / MSS?