@mattn What version of electron are you running? I downgraded to 1.4.15 electron, have v6.12.3 nodejs and 2.3.1Magic Mirror. I set the environment variable as you indicated. CPU usage goes down remarkably, although not as much as in your case.
Did you also got rid of a memory leak? I start with 43% free and this falls to 7% after a day. Stays between 5%-7% for a few days and then after 3-5 days my mirror halts completely with E_NOMEM panic in the log files. Any reader knows of handy memory leak analysistools for nodejs? It seems you keep on having 50% used / 50% free… Not me ;(.
top - 20:03:02 up 21:14, 3 users, load average: 0,30, 0,33, 0,41
Tasks: 165 total, 2 running, 163 sleeping, 0 stopped, 0 zombie
%Cpu(s): 8,6 us, 4,5 sy, 0,0 ni, 86,9 id, 0,0 wa, 0,0 hi, 0,0 si, 0,0 st
KiB Mem: 949580 total, 907688 used, 41892 free, 60200 buffers
KiB Swap: 102396 total, 5172 used, 97224 free. 444532 cached Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1178 pi 20 0 828888 244624 65840 R 22,5 25,8 327:41.98 electron
1216 pi 20 0 603272 349144 290184 S 13,6 36,8 132:11.73 electron
666 root 20 0 200848 58756 28796 S 12,2 6,2 131:39.07 Xorg
24453 pi 20 0 5224 2580 2148 R 1,0 0,3 0:02.03 top