Running the OS/apps off an SSD connected via USB3?

I’ve got the chance to buy a brand new 2015 iMac at a REALLY good price, full warranty etc. It’s got a crappy 1tb HDD not an SSD. I can’t open it and drop in an SSD as that’ll invalidate the warranty.

So, thinking of using an SSD in a USB 3 enclosure.

What sort of speed can i expect compared to having the SSD internally instaled?

That’s an interesting preview picture actually 😂

Not sure if USB3 will be good enough. Certainly using a Thunderbolt connection to an external boot SSD is workable and fast (so I have read). Last time I looked, TB enclosures were stupidly expensive.