I don't know how to fix a frame rate slow down caused by v-sync, unfortunately, but here's some stuff you might try.
1. adjust your graphic card settings:
"select performance mode under Nvidia control panel and run the game at default GPU speeds without overclocking." -Light-Yagami
https://linustechtips.com/main/topic/858108-need-for-speed-payback-on-1070-fps-issues-need-help/
(This is a suggestion for Need for Speed Payback targeted at Nvidia 1070 users, but maybe since the game is similar, the same suggestion might help.)
2. match your monitor refresh rate to the game refresh rate: (I don't know if this will help because I have a different graphics card)
a. monitor refresh rate settings:
>using Windows 10, right click desktop
>click "Display Settings"
>click "Display adapter properties" (at the bottom of the settings page)
>click tab: "Monitor"
>select a refresh rate from the drop down menu (I use "60 Hertz")
b. game refresh rate settings:
>settings >display >screen resolution (I use 1920x1080 60Hz") >right is 59Hz and left is 50Hz for me
You could try different game refresh rates with different monitor refresh rates, but I feel like that would be frustrating and might not fix anything
3. if there is no fix for you, I would just use whatever settings make the game playable enough to accomplish your goals, like only turn on v-sync for the winter stage if the game doesn't slow down to an unplayable degree, or combine that with turning your graphics settings all the way down for the winter stage to help the frame rate
I might get back to you on this topic at no appointed time in the future, if this problem effects me, because I plan to also play this game using a GTX 1080 Ti on another computer, so thanks for the heads up, and let us know if you solve your issue and what fixed the problem if you have the spare the time