why is there no System performance rating in windows 10?

If you mean the Windows Experience Index, this feature was removed starting with Windows 8.

You can still get the Windows Experience Index (WEI) scores in Windows 10.

Perform the following. 

1.Press the WinKey+S to open File Search.

Type  cmd.exe.

In the Results, Right Click  cmd.exe and select the Run as Administrator option.

In the Command Window, type the following command. 

winsat prepop 

Press Enter

This command will take a few minutes to complete, be patient. 

After the command completes, close the Command window.

2.Press the WinKey+S again and type Powershell.exe. In the Results, Right Click  Powershell.exe and select the Run as Administrator option.

In the Powershell window, type the following command.

Get-WmiObject -class Win32_WinSAT

(Note the spaces in the command)

Press Enter.

This will display the WEI results. 

What the numbers mean.

CPUScore = Processor 

D3DScore = Gaming Graphics 

DiskScore = Primary Hard Disk 

GraphicsScore = Graphics 

MemoryScore = Memory (RAM) 

WinSPRLevel = Base Score