Add note about performance, based on feedback in the PR.
Add note about performance, based on feedback in the PR.

MSeifert04/astropy,saimn/astropy,aleksandr-bakanov/astropy,joergdietrich/astropy,funbaker/astropy,DougBurke/astropy,tbabej/astropy,saimn/astropy,funbaker/astropy,saimn/astropy,funbaker/astropy,AustereCuriosity/astropy,StuartLittlefair/astropy,bsipocz/astropy,mhvk/astropy,mhvk/astropy,StuartLittlefair/astropy,kelle/astropy,lpsinger/astropy,larrybradley/astropy,astropy/astropy,joergdietrich/astropy,astropy/astropy,pllim/astropy,tbabej/astropy,StuartLittlefair/astropy,dhomeier/astropy,mhvk/astropy,larrybradley/astropy,funbaker/astropy,DougBurke/astropy,lpsinger/astropy,saimn/astropy,StuartLittlefair/astropy,dhomeier/astropy,astropy/astropy,joergdietrich/astropy,larrybradley/astropy,StuartLittlefair/astropy,AustereCuriosity/astropy,AustereCuriosity/astropy,stargaser/astropy,kelle/astropy,dhomeier/astropy,lpsinger/astropy,MSeifert04/astropy,AustereCuriosity/astropy,bsipocz/astropy,DougBurke/astropy,kelle/astropy,pllim/astropy,kelle/astropy,stargaser/astropy,AustereCuriosity/astropy,lpsinger/astropy,tbabej/astropy,MSeifert04/astropy,dhomeier/astropy,stargaser/astropy,astropy/astropy,joergdietrich/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,kelle/astropy,bsipocz/astropy,saimn/astropy,MSeifert04/astropy,pllim/astropy,pllim/astropy,joergdietrich/astropy,DougBurke/astropy,pllim/astropy,tbabej/astropy,astropy/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,mhvk/astropy,mhvk/astropy,dhomeier/astropy,stargaser/astropy,aleksandr-bakanov/astropy,tbabej/astropy,bsipocz/astropy,larrybradley/astropy