Why is my Canon’s liveview autofocus useless in low-light?

First of all I came across these questions already:
My viewfinder won’t autofocus but live view focuses well
Canon 600D Autofocus not working when using view finder but working in live-view
Why is my camera focusing fine in liveview but getting it wrong with the viewfinder?

And they really surprise me because it’s basically the opposite of what I experience. I’ve tried the Yongnuo EF 35mm and the Canon EF-S 10-18mm IS STM, and with both I experience the same: Using the optical viewfinder, I always have blazing fast, accurate AF. However using liveview, it is most of the time impossible to get a focus point at all, forcing me to move around and what not.

The camera is a Canon 70D, supposed to have a good AF system.

In a bright light condition both work perfectly fine so I doubt it’s a defect, but in lower light situation I can never get a focus point with LiveView at all – No matter which AF method I select. This surprises me as the answers to the questions I’ve linked suggest that in low light, the LiveView AF should work even better. Does my camera and its dual-ISO make a difference? If anything I’d assume it’d make it better.

Especially the video AF (which I assume does always use LiveView’s CDAF?) for the 70D is praised to be outstanding, compared to its previous models. Again: Can’t confirm. I haven’t tried video in bright light (I’d assume that’d make it better though), but in low light (it’s not that low by the way, it’s a single, but bright and close-to-subject lightbulb spotlight) the AF in video is just as bad.

Any explanations/insights/tips/tricks for a DSLR newbie like me?