Pairwise compute-time with truncated ballots?

[Steve Eppley]

If heavy ballot truncation is likely when there are a lot of
candidates, and if x=0 in the p() + xq() formula, then the pairwise
tally time might drop considerably.

For example, if each voter left unranked 80 of 100 candidates, then
only 1790 (= 190 + 20*80) counters would be incremented per voter.
Nearly 5000 counters would be incremented if all 100 were ranked. 

Or does it take more time to determine each of the 20*80 than it
does to determine each of the 190 explicit pairings?  Can anyone 
suggest an algorithm which would be fast with heavy truncation?

--Steve