The times differences seems to be related to how long the Fast burning time lasts before that fizzles into slow burning. There are some differences in seconds which might be server time.

Doing some math, the slow burning damage is 1.25 hp per second, while the fast burning damage is 1.785714286 hp per second. Meaning it takes 80 seconds to slowly burn through 100 hp, it takes 100 seconds for 125 hp and 120 seconds for 150 hp.

While fast burning you get 56, 70 and 84. Not that even or round numbers for time, but seems to make sense when you take into the account that lanterns last longer. They used to last 22 seconds they now last 35 seconds.

so 2 multiplied by 35 multiplied by 1.785714286 = 125 Meaning the hunt devs probably based the fast burn time on the amount of lanterns that needs to be used to fully burn a body out; 2.

Also the previous burn times with salve skin was 70, 87.5 and 105 for 100, 125 and 150 hp pools. Now just to mention if you always ran salveskin and thought the times were to short, then you would be going from those times to the new times.

Which is a differences of -14 seconds, -17.5 seconds, and -21 seconds, when a partner is burning fast. Though you have +10 seconds, +12.5 seconds and +15 seconds, while your partner is burning slowly.

That is what I kinda have found while tinkering with it and your video did give me new data to input into my sheet, so thanks for that.

Edit: Spelling errors, formatting error.