dslr and mirrorless lenses

Good day, I'm looking for a new prime lens for photo and video, and have decided between two lenses. The Canon RF 35mm f/1.8 STM and the Tamron SP 35 mm f/1.8 Di VC lens. Supposedly, the Tamron has better build quality and image. But considering how its a dslr lens wouldn't it be far slower compared to modern day stm af motors? They are both priced the same via used market.