[HELP] Hardsub conversion slows down video

[LuCa4S]

Hello.

I want to ask, what I should do to properly set conversion options to hardsub my video in avi file. I mean I know what to do and it works, but after conversion my video have subs, but it play little bit slower, as if it lost some fps. When I don't add any subs to conversion, everything is great and video is playing smoothly. 

I tried:

• change fps video options, 
  
• speed options, 
  
• fps subs, 
  
• convert subs into another format ass <-> srt, 
  
• convert various video files with subs
  

All don't help. I using the latest version of hybrid and it's set in default options.

Pls give me some advice. 
Sorry for my english, hope it isn't that bad.
Greetings