It's the same for me. Fastest possible rendering means slower everything else.

You have a choice... allow the render to use as much CPU as it can; or limit the resources available to it so that there is always something reserved for doing other stuff.

You could try reducing the number of threads to 3 or 2. This should reduce the CPU usage and other programs should become more responsive. I think.

Regarding the crash... how complex is your scene, and what bucket size are you using?

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)