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@Slash_Alex I had FixSpeedo speedometer, turn signals, added mods folder with custom cars and euphoria ragdolls, but even after removing them (pretty sure I removed them, cause then I went on playing MP), it still lagged.
I've been trying to get this updated trainer to work for about half a year, coming on the forums and here on and off to see if the lag is fixed (even though it turns out it was something wrong with my installed GTA).
Too bad I didn't backup the files in my root install folder, maybe I could've pinpointed the file/files that caused the lag.

@Zemanez Hey, the trainer finally works!
What I tried and failed:
Wait 15 minutes after loading (fps went as low as 1 fps)
Changing graphics presets to the lowest.
Moving GTA folder to an SSD.
Uninstalling/Disabling both my AV and Windows Defender.
Underclocking my GPU.
What worked:
Reinstalling (and redownloading) the game, even though it took 5+ hours. I had this game installed since it's release date, so maybe some crucial file was replaced and which had an impact for this specific trainer, don't really know.

Seems that both gpu and cpu work slower:
http://i.imgur.com/O39QrXS.png
Some specs if that helps:
i5-4690k (not overclocked)
gtx 970 (overclocked)
8GB ram
GTAV installed on HDD
Windows 10

As with some people here, this his version lags on my pc, while the update 28 doesn't.
While playing with 28 update, I get 60 fps and about 80% gpu usage.
With the latest update, I start with 30 fps and 28% gpu usage and it slowly ticks down to 2-6 fps with 4% gpu usage. My GPU is a msi gtx 970.
Is there any way I can help you find the problem? For example: try some tests or send some logs?