Parallelism isn't a good idea when scaling down, nor is concurrency often. Going faster is still a good idea on phones (running the CPU at higher speed uses less battery because it can turn off faster) but counting background services there is typically less than one core free, there is overhead to threading and asyncing, and your program will go faster if you take most of it out.
Except ARM CPUs aren't any more parallel in comparable power envelopes than x86 CPUs are, and x86 doesn't seem to have any issue hitting large CPU core counts, either. Most consumer software doesn't scale worth a damn, though. Particularly ~every web app which can't scale past 2 cores if it can even scale past 1.