Why do I take a lot of time to implement sth by myself.How can I improve on that

I have been working as a developer for 2 years now. I have no problem writing logic and doing programming. I have fairly good understanding of data structures .I participate in hacker rank,leet code etc without much difficulty.But when it comes to implementing sth alone, I spent a lot of time face lot of questions and confusions.

For example , I was trying to download files from google drive programmatically in python. When I started reading the documentation of google drive API, I couldnt make sense of everything I read quickly. I have to keep searching ,going back and forth, read the same sentence mutiple times etc.Why does this problem arise? What do I lack? Does it have anything to do with theory related to CS or is it sth everyone face? How can I improve?