So, in our project we are using https://bundletransformer.codeplex.com/ to process LESS files. The problem is that in debug mode (on developer machines) it works soooo sloooow. I understand, that for production is doesn't matter, but it makes life of developers much harder. Some details:
In web.config we have the following line:
<add name="LessAssetHandler" path="*.less" verb="GET" type="BundleTransformer.Less.HttpHandlers.LessAssetHandler, BundleTransformer.Less" resourceType="File" preCondition="" />
So, the answer is this:
in web.config this thing has setting, which enables/disables cache. Just turning cache on solved the issue.
<assetHandler disableServerCache="true" disableClientCache="false" />
<assetHandler disableServerCache="false" disableClientCache="false" />