I did this and it helped but its still way too slow. I followed a post here saying to check/uncheck (can't remember) download headers but I don't see that as an option in Office 365. It takes forever to sync folders causing me to have to wait and sometimes miss emails because they arrive much later than expected. I have a personal outlook account and it has a good amount of folders and does not exhibit this behavior so it seems to be that outlook should be handle this other account. They have items that cannot be deleted and I don't want to archive them.
It has a good 40 folders of which I use 5 or 6.