It could be done quickly if there were a carefully prepared contingency plan in place in which all the necessary microsurgery is anticipated. But there isn't, presumably because nobody who might have prepared such a plan believed before the referendum that it could turn out as it did. In its absence, it's got to be done slowly and carefully, with proper parliamentary oversight, if those negative outcomes are to be avoided.
There is going to be a greate deal of pain of the dull ache variety and the continued slow deterioration if England does not make the break rapidly because the Brexit will be stymied and England will be tied much more tightly to EU. The best calculated solution is not available. The politicians are almost universally determined to stay in EU and will use the delay to make it happen.
That’s like claiming the ties linking Britain to the continent prior to the Reformation were intricate too.