Concat perf
fixes #11958 

pls add a whatsnote in v0.18.0 on perf section.

squash as well
 Done, please let me know if you need anything else.  Curious, any ETA yet on 0.18.0? (No rush)
 thanks long when green

0.18.0 prob end of the month
 All green.  Thanks @jreback 
 merged via https://github.com/pydata/pandas/commit/3667b7636da878427c72dd59e1a5845e52ea306c

thanks!
