Rockstar has responded to performance issues that were reported after the release of the new patch with part two of the ILL-GOTTEN GAINS update for Grand Theft Auto V. They don't offer any solutions, yet, but this post from last week (thanks reddit via PCGamesN) says they are looking into things:

Question: After updating GTAV to 1.28 on PC, I’m noticing considerably lower framerate. Is this issue being looked into?

Answer: We have received reports of lower framerate in GTAV and GTA Online after Title Update 1.28 on PC, and we are looking into these reports now. If you would like to receive an automatic email notification as soon as there is more information, please log into the Support Site and click Subscribe at the top of this page.