yea I think the main issue is the creativity/ingenuity part and actually thinking out the problem but it is better to take a lot of time than to rush through things as important as EM
Usually, for me, understanding the chapters themselves is not the issue - that tends to go by fast. The long and laborious stuff comes when doing the problems.
Purcell has tougher problems that really reinforce the concepts but keep in mind that it is a first year text. It wont cover more advanced things, in particular the entire second half of Griffiths text. You need Griffiths for the more advanced material.