Mac Mail has very slow to send/receive mail (plus other bugs too)

It's been worse since Yosemite.

From the same exchange account (office365) I will send and receive emails instantly on my phone, but they will take several minutes through the app.

Loading existing conversations also often takes a long time.

Conversations often get jumbled up and emails that don't belong will be added to a conversation.

And finally accepting event invitations is a little messed up. I click accept and nothing happens. Then a few minutes later the email will dissapear and the event will appear in the calendar.

I am having these issues on both my laptops.

I've tried offlining and enabling the account. Also tried reindexing.

Does anyone know if these are just Yosemite bugs? Or is there somethinng I can do to fix it?

Thanks!