SamT,

I created a new template containing a single table then ran this code:

Sub Test()
Dim oTbl As Table
Dim i As Long
Dim oRng As Word.Range
For i = 1 To 500
Set oRng = ActiveDocument.Tables(1).Range
ThisDocument.AttachedTemplate.BuildingBlockEntries.Add Name:="New BB Entry" & i, Type:=wdTypeAutoText, Category:="General", Range:=oRng
Next i
End Sub

The macro took about 45 seconds to run, and I was left with 500 building blocks (all the same of course) which I could select and insert into any document. A similar process could be used to create 500 unique building blocks and is should work equally as well.

I don't think "several hundred" is a critical factor. How the OP intends to use the data, e.g., insert it, read it, etc. is probably the significant factor.