It's a rather controversial topic to discuss about. The thing about block interval is that the security of the network is taken into consideration. With a faster block, there is a risk that the propagation of the block within the network is too slow and that miners ends up mining a lot of orphans and causing more forks. Since Satoshi never publicly stated his rationale for the 10 minute block interval, it is widely assumed to be arbitary. Arguably, with better internet connection right now, the block interval can be lowered but there are still quite a few concerns.
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.
How about block orphan rate? It's biggest concern when we're talking about fast block time interval. The security depends on total hashrate and whether there are any bug which allow 51% attack at lower cost (e.g. Verge 51% attack), not block time interval.