These need not take long once you understand what you are doing and why. Preparation involves learning about the speaker, the topical nature of the subject and the audience's interests and concerns. Get speaker information from the speaker. Get audience background from members of the audience and subject information from the Program Chair, the related current awareness media or your own pipeline. When you ask for information, ask what the speaker would like you to emphasize or what the speaker thinks is relevant.
Some seasoned speakers prefer to write out their own introduction.
If a speaker provides a lengthy vitae, do not feel obligated to use it all. Shift through and pick out the things that connect the speaker with the subject and audience. Write out your introduction. Practice it in front of a mirror or into a tape recorder for timing. Hone it to sound natural and enthusiastic.