No, the network won't be less secure with 5 minute block time. I don't really think so, for example Digibyte has 15 second block time, but it is very secure, proof of work, I have never heard it was hacked, 51 attacked, reversed or forked. The point is not solely the security in my opinion.
51% attack is not about the block time. As long as you have 51% of the total hashing power, the attack can be executed.
If you observe, exchanges mostly requires those coins with a faster block interval to have a higher confirmation than those who don't. Reason being, forks are more prone to occurring with faster block times than a coin with a slower one.