To be absolutely honest, you cannot be for real to expect an undergraduate or even someone in the second/final year of their masters degree to join the firm and hit the ground running or even compare them with your full time experienced employees who have most probably had years of experience under their belts.
It seems that what you needs is a team of fully trained, highly educated robots.
I managed the interships and volunteers for a UK national charity.
How do students and trainees compare to full time staff? Clearly they are students and trainees usually with no onsite experience, so in my experince many years behind your full time staff experience.
Do they produce quality work in the first few months? Depends on the intern and whether or not you set them up to fail and what the meaning of quality work means.
Interns only have an academic experience of their chosen field not an onsite profit driven or anxiety provoking environment.
It takes time for interns to blossom