I too have a ton of DS18B20's and was floored by how long it took to get data back down the line using contrib-ds18b20-sensor
So I started diggin how is polling the sensors on a 1 wire can geting so congested
If you look at the contrib-ds18b20-sensor code ds18b20 github. You will notice that its polling information from all the sensors then checking an array for the matching ID you entered into the node.
So if you have 10 sensors its polling 10 times! 10 sensors Its not using the match rom commands found in the 1-wire documentation. DS18B20.pdf
My solution was to poll once and sort the data myself. I've had 14 sensors talking fine!
Here is my flow I'm using for one 100 yard lane of the company freezer ( oh there are more lanes 
)
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Hope this gets you to 5 seconds. 15 is more realistic.
