How do I turn it off?

I see. Is there any setting in Opera that can limit the size of those sessions? Like limit the period of the sessions saved so that those files stay at a minimal size?

Thanks! It works!! BTW, could you explain how deleting those 3 session files in the profile folder fixes this?

Hello,
I've been using Opera browser for years and when Opera changed to Chrome engine, everything was ok except for one major problem.
My Opera browser starts to lag over time. When I watch videos on youtube using the HTML5 player, the video images freeze for a while at random parts although the audio still plays perfectly. Videos still play perfectly with the Flash player so I cannot use the HTML5 player on Opera. When it gets worse, the browser's responsiveness decreases. For example:

1. When I click on a textbox and start typing, it takes a while for the cursor to appear and the text to appear.
2. When I hover over a link/button, the cursor takes a while to change to the 'hand' cursor.
3. When I roll the roller on my mouse to scroll a window, it would freeze a while before it scrolls.
   When I first installed it, there were no such problems and everything ran smoothly. When signs of lag started to show, I tried enabling/disabling the hardware acceleration option in the setting, enabling/disabling the opera plugins, switching on and off some extentions etc... but nothing worked.
   The only way to solve this is to uninstall and then reinstall Opera browser. This is impractical because if this is the only fix, I'd need to reinstall my Opera browser every few months.
   My operating system is Windows 8 x64.
   Does anyone know what's causing this? Please help.

@blackbird71 Yes, of course I'll take note before it disappears. This registry key isn't for Avira. It's a window's registry key. Setting it to '1' turns the off the function. Setting it to '3' turns it on whereas '2' makes the system run scan loosely.
It seems that after setting it to '2', Avira still scans properly.

I tried uninstalling and reinstalling opera 20 and I still couldn't download anything. Then I did another search on the internet hoping to find out if other people were experiencing the same thing, not just with Opera but, with other browsers like Firefox, Chrome and IE. In one discussion, the problem was actually with the antivirus scan registry key settings. It was fixed by editing the registry keys on the computer. Here's the discussion:

https://productforums.google.com/forum/#!topic/chrome/yVKQ4py2LwI

I tried that fix and I was able to download stuff and save images. So it wasn't a problem with Opera nor was it with Avira. It was just the OS's setting for when to do an anti-virus scan.

Anyway, thanks for the replies and support.

I only have Real-time protection option enabled in my Avira installation.
As for the config, I was talking about the setting in Opera 20 which are similar to what is seen in Opera 12 when I type "opera:config" in the address bar.

@Blackbird71 I tried turning off Avira's real-time protection and then saving an image but the same thing happened. I then tried doing this with Firefox and Opera 12 but nothing like this happened. Maybe there is a specific config in opera 20 that needs to be set?? Has anyone experienced this while using Opera 19 or below?

Trying this link from Dropbox.
https://www.dropbox.com/s/pwuuvbmgci44vq9/cannotdownload_Opera.jpg

This time I'm linking from Flickr.