I2C seems slow between Leonardo and Uno

The idea behind the final system is that a large group of people could each have an RFID tag. There's a mixture of tag numbers between 1 and 10. When a person swipes over the receiver, I read the number from the tag, and then send the value using OSC through an Ethernet shield.

I can send values using OSC, so I'm happy from that side. I can scan Numbers from the tag, so I'm happy on that side. In a perfect world, I would stack both of the shields into 1 Arduino, which would save the whole i2c hassle. I couldn't get both shields to play nicely, so this was meant to be a simple way around it!

Here's a link to a previous post of mine about the same project, for a bit of background.

Thanks for your help so far!