MicroSD card performance during boot I have a Nook HD+ and a Nook HD.  

When I ran CyanogenMod off of a MicroSD card, the Class I (43MB/s) doesn't even boot!  I spent $30 on a 32GB one.

The Class 10 (10MB/s) 16GB booted, but ran poorly

The Class 4 (4MB/s) 16GB ran perfectly.

A co-worker said that there were problems with newer classes and the phones / tablets. 

My question is: Why is this so? Is there a fix for this? What are the recommendations for good-performing MicroSD cards?

My co-worker said Sandisk Class 4 were the best, of which I have one more to play with.
The issue with most class10 sdcards for most devices is it's higher clock speed

A normal microsd (class6-below) operates only @ 0-25MHz compared to
A class10 microsd (UHS-I-UHS-II) operates @ 0-100MHz (up to 208MHz for UHS104)

This messes up with the controller on the card reader of your Nook and mistimes the writing and reading on the card making its performance appear worse. Some class10 sdcards adapt to slow controllers but I think most class10's are design for the newer. 

I still have my xperia play and I use sandisk's 32GB UHS-I class10 card. The card's performance was terrible until I got across a hack to the TI card reader controller I found on xda forums. I flashed the custom driver which fixes the timing issue on newer card and now I can get up to 12MB write/ 25MB read, no write drops, and makes a it a great swap partition.

Try to find a custom driver hack for your nook, but you will be taking big risks because you will be modifying a very low-level part of your device.