I've found out when Developer mode is enabled with following settings
Magento 2 works really slow. Please explain why it is happens.
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Developer mode is slow per se since it's intended to be used only for development. If you are looking for a speed boost you should be using production mode.
In this mode:
Any of these features might be causing what you see and most important, merging and minifying files doesn't make much sense in this scenario because you would want to have your files fresh since you are modifying them while developing.