[AIRFLOW-1729] improve dagBag time
[AIRFLOW-1729] improve dagBag time

Closes #3171 from q2w/master

andyxhadji/incubator-airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,apache/airflow,lxneng/incubator-airflow,Tagar/incubator-airflow,sergiohgz/incubator-airflow,cfei18/incubator-airflow,criccomini/airflow,wndhydrnt/airflow,cfei18/incubator-airflow,lyft/incubator-airflow,lyft/incubator-airflow,Tagar/incubator-airflow,wooga/airflow,r39132/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,Fokko/incubator-airflow,sid88in/incubator-airflow,r39132/airflow,Fokko/incubator-airflow,lxneng/incubator-airflow,apache/airflow,Tagar/incubator-airflow,danielvdende/incubator-airflow,r39132/airflow,akosel/incubator-airflow,RealImpactAnalytics/airflow,spektom/incubator-airflow,sid88in/incubator-airflow,adamhaney/airflow,sekikn/incubator-airflow,bolkedebruin/airflow,yk5/incubator-airflow,Acehaidrey/incubator-airflow,malmiron/incubator-airflow,sergiohgz/incubator-airflow,Acehaidrey/incubator-airflow,wooga/airflow,airbnb/airflow,Fokko/incubator-airflow,sid88in/incubator-airflow,r39132/airflow,wileeam/airflow,dhuang/incubator-airflow,DinoCow/airflow,wolfier/incubator-airflow,danielvdende/incubator-airflow,mrkm4ntr/incubator-airflow,nathanielvarona/airflow,airbnb/airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,nathanielvarona/airflow,dhuang/incubator-airflow,mrkm4ntr/incubator-airflow,artwr/airflow,danielvdende/incubator-airflow,owlabs/incubator-airflow,criccomini/airflow,fenglu-g/incubator-airflow,malmiron/incubator-airflow,edgarRd/incubator-airflow,apache/incubator-airflow,DinoCow/airflow,wileeam/airflow,andyxhadji/incubator-airflow,cfei18/incubator-airflow,mtagle/airflow,andyxhadji/incubator-airflow,jgao54/airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,edgarRd/incubator-airflow,apache/incubator-airflow,CloverHealth/airflow,spektom/incubator-airflow,Acehaidrey/incubator-airflow,RealImpactAnalytics/airflow,sekikn/incubator-airflow,subodhchhabra/airflow,RealImpactAnalytics/airflow,DinoCow/airflow,CloverHealth/airflow,akosel/incubator-airflow,Tagar/incubator-airflow,mistercrunch/airflow,apache/airflow,apache/airflow,Fokko/incubator-airflow,artwr/airflow,sid88in/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,akosel/incubator-airflow,danielvdende/incubator-airflow,airbnb/airflow,apache/airflow,lyft/incubator-airflow,sergiohgz/incubator-airflow,Acehaidrey/incubator-airflow,CloverHealth/airflow,apache/incubator-airflow,criccomini/airflow,bolkedebruin/airflow,malmiron/incubator-airflow,wolfier/incubator-airflow,andyxhadji/incubator-airflow,nathanielvarona/airflow,wndhydrnt/airflow,wooga/airflow,akosel/incubator-airflow,criccomini/airflow,wndhydrnt/airflow,artwr/airflow,mtagle/airflow,wileeam/airflow,apache/airflow,subodhchhabra/airflow,Acehaidrey/incubator-airflow,CloverHealth/airflow,wndhydrnt/airflow,fenglu-g/incubator-airflow,yk5/incubator-airflow,spektom/incubator-airflow,airbnb/airflow,mtagle/airflow,apache/incubator-airflow,yk5/incubator-airflow,edgarRd/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,adamhaney/airflow,artwr/airflow,edgarRd/incubator-airflow,subodhchhabra/airflow,sekikn/incubator-airflow,adamhaney/airflow,sergiohgz/incubator-airflow,fenglu-g/incubator-airflow,gtoonstra/airflow,spektom/incubator-airflow,jgao54/airflow,fenglu-g/incubator-airflow,nathanielvarona/airflow,yk5/incubator-airflow,wolfier/incubator-airflow,jgao54/airflow,cfei18/incubator-airflow,DinoCow/airflow,jgao54/airflow,malmiron/incubator-airflow,subodhchhabra/airflow,mtagle/airflow,bolkedebruin/airflow,gtoonstra/airflow,gtoonstra/airflow,wileeam/airflow,nathanielvarona/airflow,wooga/airflow,gtoonstra/airflow,mrkm4ntr/incubator-airflow,wolfier/incubator-airflow,lxneng/incubator-airflow,mistercrunch/airflow,sekikn/incubator-airflow,adamhaney/airflow,RealImpactAnalytics/airflow,owlabs/incubator-airflow,bolkedebruin/airflow,lxneng/incubator-airflow,mrkm4ntr/incubator-airflow,dhuang/incubator-airflow,mistercrunch/airflow