IS Boxer is a bit more of a drain on system resources then Keyclone.
I noticed this too, when I switched.

Basically KC renders everything at the displayed resolution.
While ISB renders everything at the resolution of the large window.

For my old system, both were easily playable.
Q6600, 8GB 800mhz ram, 4870 Radeon, 150GB Raptor HD.
AV while mounted or Epic Flight could break follow at times, but not often.

With the new system, it's smooth except for Dalaran.
Have not tried KC with the new system.
i7 920, 12GB 1600mhz ram, GTX275 Video, 64GB Patriot SSD.