PhantomJS performance

My question is, what is the most efficient configuration for PhantomJS tests.

Currently I have 2 instance of PhantomJS running and every instance can have 100 tabs opened.

Is it better to have more instance of phantomjs or opened tabs, and if tabs, what is the upper limit of PhantomJS.

CPU:

processor   : 0
vendor_id   : GenuineIntel
cpu family  : 6
model       : 58
model name  : Intel(R) Core(TM) i5-3210M CPU @ 2.50GHz
stepping    : 9
microcode   : 0x19
cpu MHz     : 2494.316
cache size  : 3072 KB
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx rdtscp lm constant_tsc up rep_good nopl xtopology nonstop_tsc pni pclmulqdq monitor ssse3 cx16 sse4_1 sse4_2 popcnt aes xsave avx rdrand hypervisor lahf_lm
bogomips    : 4988.63
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:

Memory:

           total      used       free             shared  buffers     cached
Mem:       2515896    1155828    1360068          0       171648      622668