Is it normal for my audience to see the live stream 20-30 seconds behind my presenter screen?

Yes, this is perfectly normal.

In order to provide the best streaming experience for audience members, BrightTALK must run data integrity checks on the stream before we can send it to the audience. During this process, we ensure that the quality of the presentation is at its highest so viewers can enjoy the best viewing experience. 

Rest assured that because of this check, the video and auditory components of the stream are synced perfectly together and the audience does not perceive this delay.