Were the slo mo clips recorded in slo mo or were they 60 fps clips that were automatically slowed down to match the speed of 30 fps clips in the earlier part of the movie, because you have "apply slow motion automatically" selected in the iMovie preferences? If so, you can try going to iMovie/Preferences and deselect the automatic slow motion preference. Then close the project and create a new project. Then go back to your old project and do and Edit/Select All, Edit/Paste, of the media in your project and paste the media into the newly created project. The edits will carry over. That will create a project without automatic slo mo. See if that cures it.
Otherwise, you can select the slo mo clips as a batch in your project timeline, and then click on the speed icon (the one that looks like a clock) in the toolbar at the upper right of your screen. That will reveal controls that let you adjust the speed of the selected clips. If they contain audio, there would be some distortion and so you would need to select the Preserve Pitch function in the speed controls.
If you have background audio, changing the speed of the clips would throw the audio out of sync. You would need to make adjustments.
Your way of breaking off the piece that's normal speed, fixing the rest, and then knitting them back together would work as well. You could just select the normal clips as a batch, do an Edit/Cut to get them out of the time line, and then do an Edit/Paste to put them back in after you have fixed the slo mo clips.
Best to first make a duplicate of your project and work with the duplicate, so as to preserve your original work in case things get messed up.
-- Rich