The HDR composite image is stored in a 16-bit DNG file, but is edited in a 32-bit pipeline. This approach affords greater control using the adjustment controls within Lightroom. It is saved that way by default. I suppose if that's too big for you then you could open the file in Photoshop and convert it to a 16-bit or even an 8-bit image. But you would want to do that before you used any of the adjustments in Lightroom. Personally, I like HDR the way it works now. Things could change in the future. I have no inside information.
They load and react to editing in the develop module slowly
I feel like a broken record a little as I have been suggesting this for all slowness issues with LR 6/CC. I would suggest to try turning of graphics card acceleration in Preferences->Performance. For some people this makes quite a difference with sluggishness in the Develop module with bigger files. This is true for me with a fairly new machine even. I work a lot with large stitches that can run in the 100's of megabytes to over a GB and they take ages to load with GPU acceleration turned on. This is also true for the panoramic stitches that Lightroom generates itself as well as the HDR files. Or even more so because you can't stitch at lower than the maximum resolution with Lightroom. When turned off these files load at normal speed and are usable.