microbenchmark::microbenchmark(
r1 = c(x, recursive = TRUE, use.names = FALSE),
r2 = as.vector(as.matrix(x)),
times = 1000L)
Unit: nanoseconds
expr min lq mean median uq max neval
r1 600 700 1053.7 1000 1200 9600 1000
r2 35900 37000 41604.6 37800 42100 132900 1000
c is 40x faster on my machine, if speed is of the essence.