Question: Google Slides adds an annoying amount of compression on my imported images. I want to avoid compression so I want to upload my schematics in a vector format. I can basically export to any vector format possible.

How do I do this in Google Slides?

Answer: 
As of Apr 4th 2020, I just found an easier offline solution for this issue, especially if you have the shape already in PowerPoint.

1. Just copy the shape(s) to a new PowerPoint presentation and save it as a .ppt file.
2. Upload it to Drive. The uploaded file will open in Slides and the shape(s) will maintain their vector properties.
3. Copy the shapes from the new Slides document into the one you were working on.
As of Nov 25th 2020, I just tried the old way to open a .ppt with Google slides and the .SVG will become a .jpge in Google slides.
Somehow I cannot convert .svg into smart shapes in PowerPoint, and that's why it won't work in google slides.

I find another way, which is to upload the target SVG file to Google Drive, as a regular file.
Install "CloudConvert" into Google Drive, and give it permission to read all your documents (scary! Click on the SVG file, and right-click to send to CloudConvert.
Convert to "EMF" format and (this is crucial) save back to Google Drive, and open EMF to Google Drawings. And select the target in Google Drawing and copy it into Google Slides.

I think for now this is the only way to insert svg into Google slides.