How significant is the time lag between inertial IMU data in a inertial analisys?
It depends on your algorithm, and on whether the timing is known.
Assuming that the timing of each measurement is known, you can incorporate measurements at unsynchronized moments by executing a Kalman update step for the measurements you have, at the moment the measurements happened -- just use a measurement matrix ($H$ in most Kalman literature) that matches the measurement you have, and do your update.
This does mean that you need to use the appropriate state update equations, which makes for the filter design being a royal pain in the behind -- but it's more a matter of dotting a bunch of 'i's and crossing a bunch of 't's (and not crossing 'i's or dotting 't's) than it is a matter of the basic math getting worse.