While analyzing low performance of process of SST for PXC found that in the wsrep_sst_xtrabackup-v2 script --parallel option passed to xtrabackup on donor side and missed on joiner side where xbstream binary processing SST stream. Such way the bottleneck is on joiner side where we write data in single thread.

get_stream()
{
   if [[ $sfmt == 'xbstream' ]]; then
       wsrep_log_debug "Streaming with xbstream"
       if [[ "$WSREP_SST_OPT_ROLE"  == "joiner" ]]; then
           strmcmd="xbstream \$xbstreameopts -x"
       else
           strmcmd="xbstream \$xbstreameopts -c \${FILE_TO_STREAM}"
       fi
   else
...

Fix for the issue for wsrep_sst_xtrabackup-v2 in 5.7.28:

— wsrep_sst_xtrabackup-v2      2019-09-15 02:25:33.000000000 -0400
+++ wsrep_sst_xtrabackup-v2.fixed      2020-01-15 10:58:33.970457822 -0500
@@ -269,7 +269,7 @@
 
        if [[ "$WSREP_SST_OPT_ROLE" == "joiner" ]]; then
            # Decryption is done by xbstream for SST

•            xbstreameopts_sst=" --decrypt=$ealgo $encrypt_opts --encrypt-threads=$encrypt_threads "
  +            xbstreameopts_sst=" --decrypt=$ealgo $encrypt_opts --encrypt-threads=$encrypt_threads --parallel=${backup_threads}"
              xbstreameopts_other=""
              ieopts=""
          else