n=10000
p=1000
q=5000
A = matrix(runif(n*p),nrow=n, ncol=p)
B = matrix(runif(p*q),nrow=p, ncol=q)
C = A %*% B

> sessionInfo()
R version 3.4.1 (2017-06-30)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.3 LTS

Matrix products: default
BLAS: /usr/lib/openblas-base/libblas.so.3
LAPACK: /usr/lib/libopenblasp-r0.2.18.so

locale:
 [1] LC_CTYPE=fr_FR.utf8       LC_NUMERIC=C
 [3] LC_TIME=fr_FR.utf8        LC_COLLATE=fr_FR.utf8
 [5] LC_MONETARY=fr_FR.utf8    LC_MESSAGES=fr_FR.utf8
 [7] LC_PAPER=fr_FR.utf8       LC_NAME=C
 [9] LC_ADDRESS=C              LC_TELEPHONE=C
[11] LC_MEASUREMENT=fr_FR.utf8 LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

loaded via a namespace (and not attached):
[1] compiler_3.4.1