ID 358312 - "Join" vertices command is slow to process in mesh items containing large quantities of vertices.

Problem summary
"Join" vertices command is very slow to process and complete if the item layer has a high number of polygons. It doesn't matter if you hide the other polygons, the lag still occurs. 

Customer reported version
modo.n/a

Customer reported platform
windows10

Steps to reproduce

1) Launch Modo and open provided scene file
2) Hit '1' to enter Vertices mode and select 2 vertices.
3) In the Model layout, select the Vertex tab and select Join
Result: Process takes large amount of time to complete (avg 3-5 mins)

Workaround
Cut and paste the relevant polygons onto a new item layer.

Reproduced by support
This bug has been reproduced in:
12.1v1 - OSX 10.12.6, Windows 10
12.0v1 - OSX 10.12.6
11.2v3 - OSX 10.12.6
11.2v1 - OSX 10.12.6
11.1v1 - OSX 10.12.6
11.0v3 - OSX 10.12.6
11.0v1 - OSX 10.12.6, Windows 10

Unable to reproduce bug in:
10.2v4 - OSX 10.12.6, Windows 10

Earliest version tested
10.2v4 - OSX 10.12.6, Windows 10 - This issue no longer appears in this version and has regressed

Expected behaviour
Vertices join quickly

Actual behaviour
The "Join" process take anywhere between 3-5 mins to complete