Installing the old DirectX 9 on Windows 10 may sound weird at first, but let's see what the reason is, shall we? It will very likely help Improving a lot your SWTOR Performance and FPS.
Windows 10 comes packed with the newest edition of DirectX 12. Our favorite game SWTOR is created and still uses only Directx 9. In order for it to display its beauty, the system needs to emulate al the DirectX 9 effects, which may slow it down and result in a poor performance. By installing the DirectX 9 package manually, we stop that process of simulation and allow our PC to use the original files the game is designed for, thus, in theory at least, speeding the process up. This installation wont make SWTOR run smooth and flawlessly, but it will likely boost it at least a bit. I have done this since the game's launch in 2011 on multiple versions of Windows - Windows 7, Windows 8 + 8.1 and now Windows 10.
By doing that, you DO NOT remove the DirectX 12, no, it just installs extra libraries and gives the game you play an option which ones to use. If the game can benefit from DirectX 11 or 12, it WILL use those. There is no overriding or replacing, the installation just adds another layer.
One last note. Installing DirectX 9 does not hurt your system, it will not break anything. The link provided below is from Micro$oft's official website.
You can download the old DirectX 9 from HERE