Assume I have 30 computers connected to a network.
Is there any disadvantages in using /24 sub net mask over a /27 sub net mask?
Do the extra IP addresses (the ones that are not configured on any node) cause a negative effect on the network performance?
No, the number of hosts doesn't matter.
Operations on bitmasks such as used in CIDR subnets are very fast and constant speed. Bit operations on 32 or 128 bit integers are easy, and often hardware accelerated in ASICs as well.
As an extreme case, all IPv6 subnets are /64 in size, which is enormous. This means with IPv6 you don't have to size subnets anymore. Aggregated prefixes like the /32 assigned to a large organization are even bigger, but still it is just a bitmask on integers.
