BitDefender erroneously labeled the other thread of this issue as "Fixed", so I'm starting a new one. The problem persists even after the latest update until this date (Jan 19th, 2018).
Symptoms: The computer becomes laggy to use seemingly at random, with any moving video suffering from slow frame rate, any sounds becoming choppy and distorted, and even the mouse cursor becoming jumpy and laggy. The system continues to respond to commands but with significant delay. The problem is frequently recurring but does not appear to be linked to any particular program or activity besides being triggered by almost any process that initiates even a moderate amount of network traffic.
Problem: Opening the Task Manager show that System is constantly taking up 20-30 % of CPU resources. This is unaffected by anything that is currently being done on the computer and does not stop even if every other resource-intensive process is killed. Even if the computer is otherwise completely idle the "System" process keeps using the same amount of CPU resources and the system remains extremely slow.
A reboot fixes the problem temporarily until it reappears. Before that happens, the System process uses only 0,1 % of CPU resources in regular browsing use or when idle.
Diagnosis & Cause: I ran Windows Performance Recorder (WPRUI) while experiencing the issue and analyzed the log with Windows Performance Analyzer (WPA). Opening the System process under Computation > CPU Usage (Precise) showed that the process actually responsible for over 80% of the System CPU usage was in fact vsserv.exe.
Vsserv.exe is a BitDefender process.
Troubleshooting: Vsserv.exe is triggered into this haywire mode by innocuous network traffic that BitDefender wants to scan. However, adding programs to BitDefender's white lists or exclusions does not remedy the problem. Disabling "Block port scans in the network" does not remedy the problem. Reinstalling BitDefender does not remedy the problem. Updating to the latest version (Jan 19th, 2018) does not remedy the problem. Disabling "Advanced Threat Detection" may or may not help, but I cannot confirm that at this time.
