Wrong! This is perhaps the best time ever to hit the slush pile!
1. For every member of staff who have children, there are as many - if not more - who don't. Publishing is stuffed with people in their 20s who are as yet child-free. But, perhaps because it's a female dominated industry, there will be many senior editors who will be taking time off. Things do slow down in the summer because people are away so there's less work to do, but lots of staff will be at their desks throughout the summer months - and even if they go away, it's only for a couple of weeks. Time to hit the slush pile.
2. What do you do when you're about to leave for a couple of weeks holiday? It's only human nature to want to leave a tidy/empty desk. Editors and agents often make a big effort to clear a slush pile back log before they go off on holiday. Take advantage of it.
3. Going through the slush pile is a nice occupation for a literate young student on work experience. They can't do too much harm, and are quite capable of weeding out the complete no-hopers. When do students do work experience? Yup - the summer holidays.
If you're ready to go, don't wait until September when agents and publishers start getting ready for the Frankfurt Book Fair at the beginning of October. Get in there now!