I really don't see how this would have a major negative impact on speed. If anything, it will allow CSS files to be smaller reducing the amount of data being downloaded to the client and may actually improve performance.

> The issue is speed.

It makes no difference, these are proposal for constants so it's trivial substitution. Furthermore by avoiding repeating the same value over and over again CSS variables likely reduce the file's size.

This isn't even close to being an 'issue'.

Unless maybe you are living in the 80s and using a Spectrum 48k?

Speed. Christ.