Every 5 seconds, GTA V "stutters." The problem does not occur when disconnected from the network.
Also, of interest, The Sims 4 experiences a similar issue.
The WikiHow article is pretty good. Check for the following processes.
AcroTray
Delayed launcher
Also review Start-up.
The Autoruns utility from Sysinternals can provide a more comprehensive view.
A version of steam_api64.dll
may have been hijacked. Use the Microsoft dumpbin
utility.
dumpbin /exports C:\Program Files (x86)\Steam\steamapps\common\Grand Theft Auto V\steam_api64.dll
Start in Safe Mode with Networking. Hit F8
at Power On.
See also; How to detect if a Keylogger is installed?
Try to detect rogue programs attempting to "phone home." Unplug the Ethernet cable and disable wireless, then monitor outbound connections with TCPView.
Check for a rootkit.
Windows 10 Startup tasks.