We found this tip on Reddit's TOR community first, and it's proven enough of a help for me to pass it along to you. The game's DX10/11 seems to emulate missing libraries from DX9, which can cause some slowdown issues in certain parts. By filling the gaps with DX9's files, I noticed a smoother transition in mounting/dismounting and a much better performance in Space Missions. However, as DX9 is still used and installed on many games, you may have the libraries already.
You can find these files on Microsoft's support page here. Start by making a new folder, and when you run the downloaded .exe, extract all the files to this new folder (this will help with clutter). Then, go into your newly filled folder and run DXSETUP.exe. This will run the installer and will apply all the missing libraries to your DX9 folder.