Hi Paulm,
CPU-demanding instruments such as the Micrologue significantly increase the system load. Furthermore older devices such as iPad 2 and iPad 3 reach their capacities significantly faster than current models.
As a workaround try to find a good compromise between Microsonic (CPU-lightweight) and Micrologue instances. Avoid huge polyphonic pad sounds from Micrologue (go for monophonic sounds instead) and/or make use of track freeze to free some CPU-power.
Please follow this link to learn more about the topic:
Hope that helps,
Lars