Why has IMAP speed on outlook.com slowed down?

Since the introduction of IMAP in outlook.com, I started migrating from gmail to outlook.com. I use a third party service called migrationwiz.com to migrate and the service uses IMAP to transfer messages.  I have about 1.8 gigabytes of messages in my gmail account. First when I started migrating the transfer speed was 600mb/hour and now it has slowed down to 24mb/hour.  That's the transfer speed for outlook.com and not for gmail. Gmail still showing a transfer speed of 3400mb/hour. 

I figured that maybe, outlook.com limits the speed of IMAP for free accounts, so I upgraded my account to No-Ads outlook.com. Still the speed hasn't improved.  Does microsoft limit the transfer speed for paid accounts also?

The reason I'm asking is that it would take more than a week to migrate my messages at 24.5mb/hour. And if microsoft does indeed limit the speed by that much, I might actually not migrate to outlook.com and continue using gmail.