Using the file from the original,

    1#desc sum each group (!/) (" II";"\t") 0: `:tsvfile

Took about 3 seconds, 2.5 of which was reading the file

EDIT:

    q)\ts d: (!/) (" II";"\t") 0: `:tsvfile
    2489 134218576
    q)\ts 1#desc sum each group d
    486 253055104

I was using the first example with a char in the first column.

   A 4
   B 5
   B 8
   C 9
   A 6

How to solve with only a dict?

Regarding the 1gram file at https://storage.googleapis.com/books/ngrams/books/googlebook...

This is the result I got

   3| 1742563279

using

   q)\ts d:(!/)(" II";"\t")0:`:1gram
   q)\ts 1#desc sum each group d
   1897 134218176
   371 238872864

or

   k)\ts d:(!/)(" II";"\t")0:`:1gram
   k)\ts desc:{$[99h=@x;(!x)[i]!r i:>r:. x;0h>@x;'`rank;x@>x]}
   k)\ts 1#desc (sum'=:d)
   1897 134218176
   0 3152
   372 238872864

No doubt I must be doing some things wrong.

I actually had it wrong in mine. Wasn't paying attention and had the dictionary the wrong way around. Probably would have been more obvious with the char since you can't sum them...

With the reverse thrown in to switch the key/value around we get the correct answer

    q) 1#desc sum each group (!/) reverse (" II";"\t")0:`:1gram
    2006| 22569013

or

    k) {(&x=|/x)#x}@+/'=:!/|(" II";"\t")0:`:1gram
    (,2006i)!,22569013i

Works the same for the simple example

    k)e: 4 5 8 9 6!"ABBCA"
    k){(&x=|/x)#x}@+/'=:e
    (,"B")!,13