There is no reason to be running at 88.2khz. That right there is putting a big strain on your cpu because file sizes and data streams are huge. Set your sample rate to 44.1khz and live life happy.
Another observation, if you aren't monitoring your effects in Cubase on the track you're recording as you're recording it, you can set your buffer to a higher number. Doing so will also ease the workload on your cpu.
The 32 bit float and 64 bit float thing is not related to buffer size or anything. Mine has always been set to 32bit, but maybe 64bit is easier on the cpu because it matches the operating system. Honestly don't know about that one....