Just finished my impromptu test.  I rendered the same animation (640x480, 924 frames) twice, using the OpenGL preview engine, with antialiasing and 2D motion blur enabled.  I noticed no significant change in performance.  Both renders took just over 50 minutes.

Admittedly, my system is hardly a powerhouse (Athlon 64 X2 3800+, 2GB system RAM, with a GeForce 8400, 512MB video RAM).  Your accumulation-buffer mileage may vary, depending upon your hardware.