What is AppSC extremely slowing down my MacBook Pro?
AppSC is malware. Download, install and run the free app Malwarebytes Anti-Malware for Mac. It will detect it and you will be asked to remove it (and some other potentially installed malware).

If you can't download/install the anti-malware app because everything is sluggish, you have to move or remove the responsible plist file(s) in /Users/your_user_name/Library/LaunchAgents/, /Library/LaunchAgents/ or /Library/LaunchDaemons/ manually by booting to Single User Mode or to Recovery Mode. Remove the responsible plist file by cd'ing to one of the directories and mv'ing or rm'ing it. I don't remember precisely but I think the file name contains *.appsc.* or *.appscd.*. If you are running El Capitan you might have to disable System Integrity Protection. Then continue booting or reboot to your main volume and try again downloading, installing and running Malwarebytes Anti-Malware for Mac.



I recently got hands on a client's iMac infected with AppSC and some other malware. I've suspected MacKeeper (which was "installed" also) as the main culprit but I haven't been able to substantiate my assumption.

To remove the malware I first booted to a thumb drive containing a full OS X Mavericks and then - after investigating modify/create dates of several apps and folders - moving suspicious launch agents/daemons in ~/Library/LaunchAgents, /Library/LaunchAgents or /Library/LaunchDaemons to inert folders like /Library/LaunchDaemons (Off). Then I booted to the main volume and installed the aforementioned software and uninstalled/deleted all malware files.

AppSC (accompanied by an AppSC daemon which starts AppSC while booting and restarts AppSC after force quitting it) just "eats" RAM with a rate of 50 MB-300 MB/s. Immediately after starting the iMac about 6 GB of 8 GB RAM were free. After all free RAM is gone the whole system swaps with a slightly slower rate. After one minute the client's iMac had a swap size of 4 GB with no other apps running. 
Memory swapping will slow down your OS X and all running regular apps and after a while it's almost impossible to do anything even rebooting.