So, I did my annual deep cleaning of my Hottop-B2K.
This time, though, I did a much better job on the exhaust fan than usual.
Also, I use a stainless steel screen in place of the OEM filter which has a much lower resistance.

Then I roasted. I use Artisan with stored profiles.

I kept getting stalls or otherwise noticing that the beans were sucking up heat.
Then I noticed that the roaster was cooling down much faster after roasts.
Then the light bulb went off in my head (duh):
My fan was working about twice as well as prior to the cleansing.

I have used a stainless steel computer fan screen in my Hottop rear filter for several years. I found it necessary to replace the white Hottop fan material with one sheet of paper towel. If I did not use the paper towel (as seen below), it caused too much of a change in behavior (versus my prior roast profiles).

I routinely clean my Hottop parts that are easily removed. Then I simply replace the piece of paper towel.

I use the same screen. No paper towel. I just adjust the fan lower than before I changed to the stainless steel screen.
Once a year I do a major cleaning and this year I did a better job on the fan.

Same screen plus a cut out off any universal grease filter.It works !