Sorry to hear you are having problems after the Android 10 update,
Samantha Jones.
Which model of OnePlus 6T do you have... Global model 6T, McLaren Edition 6T or global converted T-Mobile carrier minority variant 6T?
The, "Bug Report", is in the Feedback section of the OnePlus forums, here...
https://forums.oneplus.com/feedback/
It would appear that you are not alone with this issue, see...
Slow animations, Less responsiveness on Android 10
one plus 6t performance lag after android 10 update
Both of these threads suggest backing up your phone and performing a Factory Reset.
However, before embarking on that course, it might be worth trying the following...
Ensure that, "NFS: No Limits", is not optimised in Battery...
Settings > Battery > Battery optimisation > NFS No Limits > Don't optimise
You can also enable Developer Options on your phone...
In Developer options you can increase the, Window animation scale, Transition animation scale and Animator duration scale. I would initially suggest moving them from 1 to 0.5.
Try clearing the games cache, not data...
Settings > apps & notifications > SEE ALL *** APPS > NFS: No Limits > Storage & cache > Clear cache
Enable RAM Boost...
Settings > System > RAM Boost > Increase RAM utilization
Enable Game Space and Fnatic mode for NFS: No Limits...
App drawer > Game Space > + (add NFS: No Limits > Menu (cog wheel), choose options and Fnatic mode
See if the above cures your lag. If not, A Factory Reset will be needed.[/QUOT0E]
I use a Global model 6T,
Thank you will try the above and let you know.