The main impact of a shorter process would be felt in the primary process. The most likely impact is that "dark horse" candidates would be less likely to be a factor in the campaigns and fund raising would become more important.
With a drawn-out process, a candidate has the ability to mount a campaign that starts out slowly but builds momentum as the candidate becomes better known. This process has helped presidents like Bill Clinton and Barack Obama get nominated. It also brought Rick Santorum to prominence in the Republican primaries this year.
With a more truncated process, such candidates would not be able to thrive. The candidates who succeeded would be those who could build a strong organization and raise a great deal of money well before the elections start. There would be no time for a dark horse to gain momentum. The winning candidates in such a system would be those who were already well-known and who could get large donations to finance a very short but intense campaign.