Clip retiming no longer works!

As Tom said, this is more likely to do with disk performance and space.

As you are creating BOTH proxy media AND optimized media, this takes a great deal of disk space, and is probably unwarranted. Note that Optimized media uses a LOT of space (it can very easily use ten times as much as the original). The conversion that they had you do in Finder is completely moot if you transcode on Import, as shown in your screenshot. Since you apparently have Optimized media for everything, it is those HUGE files that will be used in editing. They are lighter for your CPU but much more heavy on your drive.

You can of course switch your Viewer to Proxy and that should avoid the dropped frames, but the question remains as to WHY you'd transcode to both proxy and optimized. FWIW, my recommendation is to do no transcoding at all on importing.

So apparently you don't have proxies, or they're not where the application expects them to be.

Did you try rendering to check the performance? If that still doesn't do it post an EtreCheck report https://etrecheck.com giving the application Full Disk Access in Privacy & Security Sysyytem Settings. Copy the report and use the Add Text button to paste the result into reply box. 

It could be a lot of things related to disk space and drive and system performance. Do you have background rendering switched on? Try rendering the section with the speed effects and see what the performance is like.

If the media was on slow external drives that could certainly explain slowness.

You can greatly diminish the sizes of your libraries before transferring - see below. And, most importantly, you don’t need to have everything in the internal at the same time. You can simply copy what you are working on to the internal, and copy off to an external when you are done.

Relinking may or may not be necessary, depending on where your media is (if inside the library, it will just come across and nothing more is needed).

Before you copy: open your library in FCP, select the library in the browser and do File->Delete Generated Library Files. Check all the options. Then close FCP and drag the library to your internal drive

There are 4 things you need to do:

1. Submit an EtreChaeck report

2. Back up with TimeMachine

3. Merge the library and media and copy them to the movie folder

・Select all events in the browser

・Right-click: Copy events to library: New library.

・Use the same library name. Specify the movie folder.

・Target: Media, Check "Original media" and "Copy media stored in external locations", Media output destination: Media: Library.

4. Remove "Transcode" and "Create proxies" from the import settings

Not an easy task as a lot of file relinking will likely be required.

Follow step 3. It's easy to consolidate and move your libraries and media.

I've already copied the Libraries over the to Mini's Movies folder.

Not sure if you did it the right way. Please follow steps 1 to 4.

Drop down menu in the upper right corner of the viewer.