library(microbenchmark)
microbenchmark(x = NULL, x <- NULL, times = 100e3)

Unit: nanoseconds
      expr min lq    mean median uq  max neval
         x   0  0 0.60819      0  0 8426 1e+05
 x <- NULL   0  0 1.75101      0  1 8727 1e+05