Typically when I play on KGS I play on the "fast" automatch settings: 10 minutes plus 0:20x5 byo-yomi. It is fast, and I spend a good portion of every game in byo-yomi, but 20 seconds per move is usually enough to do some reading and not feel like I'm in instinct-only blitz mode.

However, I absolutely cannot count the board in 20 seconds. The best I can do is a very rough "that corner looks big, but this looks about the same size, maybe white is ahead" kind of estimation. I timed myself on some sgf files and to do an accurate count of the board in the early endgame takes me about 2 minutes. The way I see it there are a few things I could try to do:

1) When I have 2 minutes left on the clock, stop and take a full count no matter what.

2) Learn to count the whole board in 20 seconds. I'm not sure this is even possible, but maybe it is. If I'm willing to burn a byo-yomi I could take 40 seconds, though I couldn't do that more than a few times.

3) Count parts of the board and remember them. Maybe when I have an easy move to make I take 10-15 seconds to count the value of a corner or side, and then remember those and add up the totals over several turns of counting. Some of this could be done on the opponent's time also.

Number 1 would definitely work, but only once per game, and it burns valuable clock time. Number 2 basically comes down to "count faster", I'm not sure exactly how to do that. Number 3 seems great in theory, but I have a terrible time remember how big a territory is and end up recounting the same corner multiple times. I'm not sure I could remember enough to count the full board this way.

I know some people may say "if you want to count, play a slower game", but assume for now I want to stick with fast games: is it still possible to know who is winning?

To remember the individuel points for each territory has also the advantage that you can more easy update your count when one changes. Of corse this requieres to remember them even longer than for only a few moves but will be much faster (if you don't need to recount).

One way to accomplish it with a bad memory would be to write it down, but this has something of cheating...
To train the memory is a very trivial tip, but I cannot see a better.

In addition, in fast games count when you are still not under time pressure. Stretch the initial counts over a succession of byoyomi periods. When you need to read life and death, it is too late to count if you can sacrifice.

Using the two words counting and fast in the same sentence seems a little oxymoronic.

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Still officially AGA 5d but I play so irregularly these days that I am probably only 3d or 4d over the board (but hopefully still 5d in terms of knowledge, theory and the ability to contribute).

IMO you should assess the position (of which counting is part) three times before you go into byoyomi. That leaves you around 5 sec. per move, which is fast, but not blitz. After that, 20 sec. byoyomi should feel like a luxury.

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The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

In Endgame, Tomoko Ogawa writes she is able to count an almost-solid territory, with open skirt, accurately in few seconds. (Of course, she is a professional.) I think the harder part is to evaluate largely neutral areas.

Not for a 100% accurate count, but Cho Chikun's method in his High Speed Game Analysis book is fairly helpful in my opinion to counting a board swiftly. I can normally count within a 10 point difference in approx 10 seconds.