From my experience, it seems like the iPhone downloads only headers by default over IMAP while Mail.app may download the full e-mail. This would make syncs appear faster on the iPhone, but take longer to appear when viewed in full.
The iPhone may also be doing more work in the background that you may not realize, and just not displaying progress bars for such work, while you should be able to see all such activity on the macbook app. Logically as long as the connection speed is comparable between the two devices, and both use IMAP, the speeds should be similar.
If you're experiencing an actual slowdown on your Macbook when IMAP is syncing, there may be issues with your setup, but just comparing progress bars to judge speed may be an unfair comparison.