I’m with PL_Moon,
Speed slowdown is necessary for sentence mode across devices. It’s on the mobile apps (thank goodness), but needs to be a feature in the web browser sentence mode, as well.
For now, I copy-paste my sentences into mirinae.io and use their speed control to listen to the sentence slowed down, but it would be much more convenient to do it in the LingQ browser, as I would on a mobile device.