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I personally haven't seen very much bluffing going on in massive pots that are HU, because

A. 20BB pots are rare
B. most massive pots aren't HU

I'm not sure what the average players bluff frequency is in 20BB pots

also boards with broken draws are more likely to have a river bluff than the raggy boards, and LAGs tend to have a hell I can only win with a bluff mentality a lot

so if he's only winning 5% against LAGs he's likely too tight