I remember reading in a book about Windows 1.0 (I think it was) using a "fast mode" method of loading libraries from .ovl files, versus a "slow mode" of individ

I remember reading in a book about Windows 1.0 (I think it was) using a "fast mode" method of loading libraries from .ovl files, versus a "slow mode" of individual DLLs, which is the method still used in Windows today. Is anyone aware of more information on this history?