It's called bottleneck effect for nr2003. It's been told about 100+ times there already what to do or how to fix it. Look for topics.
As a hint I can give you that option - try to compress car files with WinMip2. Other one option- tweak graphic settings with this http://bigdonlinemotorsports.com/NRgtweak.php. If you have good (really good) HiEnd PC you can try to do this way -
Is there a way for me to go back to dx9 and have all my other games work to on Windows 10? FSE track Daytona I can run 70 to 100 fps,but turn 2 and 4 they dip down to the low 20's
First of all, NR2003 uses DX8 not even DX9 to use with DX9 you need to do what I wrote above and go trough NR2003 Graphics Update Tutorial: DX9 NR2003 Update!
Kudos to the new guy who searched the forums for his answer and read the stickies before asking the same question that's been asked 10000 times. Here's to you sir.