Zend certified PHP/Magento developer

Why in developer mode nothing works

I inherited developments made by a third party provider on our Magento instance.

When a set the developer mode nothings work anymore and it is all super slow. The layout is mess up, the buttons do not all work properly and it takes ages to load a single component.

Do I need to do some preliminary tasks (ex: delete cache, etc.) or it is caused from bad development made by the provider?

Many thanks