I am not sure what you mean when I tested a pionner squad needed 30 secs to build a whole mine field.
A Tm-35 took about 13 secs.
A CE took about 36 to clear.
The extra 6 sec to clear than to build it is not an issue imo.
I mean It took around 5-6 sec per boxes.
don't forget about you do not need full time to planted it. you can plant 1 box and comeback later.
I think setup time is not an issue because any building/set up you must be sure that your squad will be safe before you going to build it or just cover it like when UKF building their emplacement.
However
Can you explain about time to defusing not an issue?