Hearthpwn

That's interesting. Battle.net is bad, but I didn't know it was that bad. What kind of hardware are you running? Playing videos and stuff in the background shouldn't be enough to lag your game unless your graphics card is low-end or something. I have a GTX1080 and I play stuff like PUBG at like 60 - 80fps on ultra settings, so there's absolutely no way I was supposed to be getting any sort of lag whatsoever. It was also very strange to me that HS was only lagging when I was playing games. It was super smooth otherwise. That's when I did some digging and realized what was happening.

I'm glad your lag fix was simple, at least!

Hello, everyone! Sorry for this obnoxious title, but I feel like I had to get this out there to anybody experiencing this problem. What better way to do that than to be as obnoxious as possible?

So, I have been struggling with stuttering and lagging after every in-game action I have made or my opponent has made for the past few months in Hearthstone. I honestly couldn't figure out why it was happening and I couldn't find the solution anywhere. If this sounds like something you are experiencing, then here's a solution that one wouldn't expect whatsoever:

It turns out that the problem actually has to do with Windows Defender. The HS Deck Tracker creates a log file in your HS folder that gets flagged by Windows Defender and it lags the game because it thinks that said file is malicious. You can fix this by excluding the HS folder in Windows Defender. 

Step 1

Search for "Windows Defender" in your Windows start menu and click on the "Windows Defender Security Center" icon.

Step 2

Click on the "Virus & threat protection" icon (the one that looks like a shield).
Click on the "Virus & threat protection settings" text.
If you think this could be the problem, but you aren't sure, what you can do is temporarily turn off "Real-time protection" and enter a game of Hearthstone. WARNING: THIS WILL TURN OFF YOUR ANTI-VIRUS PROTECTION, SO BE CAREFUL. If you continue to have stutters and frame lag, I'm sorry, but this is almost definitely not the solution to your problem.

Step 3

Scroll down to where it says "Exclusions" and click on "Add or remove exclusions".
Click the "+" button.
Choose "Folder".
Navigate to your Hearthstone folder. If you installed it to the default path, it should be located using this path:
C:\Program Files (x86)\Hearthstone
Click on this folder and click on the "Select Folder" button. This will exclude that file path from being scanned by Windows Defender. To be safe, I have also done this with the Blizzard folder located using this path:
C:\Users\YourUserName\AppData\Local\Blizzard
This path contains the cache that HS uses to speed itself up. I also excluded this path just to be entirely safe before I figured out that the problem lies within the log file in the Hearthstone file itself. I still have this excluded because it does no harm to keep it from being scanned.

That is it! Hopefully your stuttering and such should be fixed! If you have any questions, feel free to ask and I will try to solve them for you!