Any benefit to editing videos off an external SSD

Yes.

128 gig is enough for your OS and Apps.

FCP X Projects can take lots of gigs to capture footage and also Render stuff if required.

One Library with 2 hours footage plus render files etc. can easily add up to way more than 128gig.

You need an external to run your Libraries.

Buy as big as you can afford, I'd suggest. minimum of 500gig to 1 TB.

Once you have completed your work the Library can be backed up to a cheaper spinning drive for later use if required.

USB 3 or Thunderbolt speeds will be sufficient for most video needs.

Always format your hard drives for Mac use, Mac OS Extended for spinning drives and APFS for SSD drives.

Another thing to consider is that when drives reach 20% free of their overall capacity, it is approaching full status.

Drives need free spaces to operate correctly, SSD more so than spinning drives.

Al

The information you have been given is sound.

Just to add an alternative perspective to what has already been said, the need for speed is very personal and its importance varies according to your circumstances.

A professional working to a tight deadline will require the fastest possible setup, regardless of cost whereas an amateur film maker may be able to take a more leisurely approach.

I belong to the latter category and although I have a 1TB Fusion drive iMac and a 250GB external SSD, I find myself using 2TB USB 3.0 portable hard drives most of the time as they are more than fast enough for my editing style and are incredibly cheap and have a massive capacity when compared to SSDs.

It's a matter of horses for courses and ultimate speed is not necessarily desireable for everyone.

Maybe a quick question, but it would need a long answer. A short answer is that the formatting allows caching and snapshots to be accessed, used, and dropped as needed very quickly. These things don’t work well on a spinning disk with it’s slow seek time but make it very efficient on an SSD.

For media on an SSD for use with FCP, yes. For other things maybe not.

As a small addition to what Al said: you could edit in the internal if it were larger. The internal is actually faster in most cases. With only 128GB, you absolutely need an external SSD.

But even with a big enough internal SSD, there could be good reasons to work on an external.

Here is a simple one: your internal drive is probably backed up to a Time Machine drive (if not, it should!).

You work on a video production that temporarily uses 200GB; it gets backed up to TM.

Even if you erase it when done, it will still be occupying hundreds of GB.

Your Time Machine drive will fill up pretty quickly and start dropping older backups.

I'd rather keep my video backed up separately.