It takes a bit longer for IRS to generate Tax Transcripts if tax returns are not processed by them in their system because of heavy workload or other reasons. That being said, IRS can produce Tax Trasnscripts only 2-3 weeks later of processing the tax returns in their system.
Also, it's always advisable to have the tax transcripts than actual tax returns when it comes to USCIS. Why? Because Tax Transcripts are official documents, while tax returns are NOT unless tax returns are certified by IRS which might cost money/time and efforts. As per the interm memo to all Immigration adjudicators/officers which also specificially stated that adjudicators must ask the applicants for the tax transcripts whenever there is a doubt about their submitted tax-returns. So, in order to avoid any delay or pitafall, better to have tax transcripts than tax returns because you don't know what officer could ask you at the interview when it comes to this issue of taxes.