I, and many other people regardless of graphics card, CPU, or RAM, are experiencing an issue where the game slows to a stuttering crawl after a while. Adding RAM extends the playable time period, but only staves off the inevitable. There are many lengthy threads on the topic, with people suggesting things like putting the pagefile on an SSD to ameliorate the bottleneck, or using third-party programs to aggressively clear caches. For every homegrown solution, there are people who claim it doesn't work for them. Google "gta v pc stuttering" for an overview.
