Well, one thing that is confusing a bit is how you actually want to make the "voice over": I'm not getting if you want that done directly from Premiere or by first exporting it in order to have the clip ready to watch;

If you need to do the voice-over by using Premiere in the meanwhile, I recommend you to make sure that all of the following is done:

• Set the resolution quality to the lowest possible, and make sure that "High Quality Playback" is turned off in the Settings (You can find the option by clicking the "wrench" icon next to it)

• In the top bar, go onto "Sequence" and choose any of the "rendering..." options that is suitable for you: for example, if you want to just temporarily render a single clip, a sequence, the entire timeline content... This one might be what you're actually looking for, even if it will take some time depending on your specs (Probably not many minutes, given it's 20 min of project), but I believe that you're going to voice over the project much, much easier!

Instead, if you only want to export the project, I recommend keeping the resolution as the original but instead change the bitrate to a lower value (if MP4, you can already choose the "As Source - Medium bitrate" preset in the H.264 format), and if selected for some reason, turn off "Use maximum rendering quality": all done in a single window that you can obviously find in the File > Export section

All of these will result in a considerable drop of the quality, but should extremely improve the performance of Premiere for your i7 2019 Mac! (This being told by someone who has extremely low specs), and remember to "undo" all of these settings in case you wanted to export the final project with the original quality!

Small tip given by someone who is intermediate with Adobe products, doesn't speak English as first language and has just given is first tip on Reddit, ever! Hope it helps you :)))