Does an rj-45 coupler reduce network performance?

The RJ-45 coupler has nothing to do with the increase or decrease of performance in a network. In fact, this is a requirement to connect two ends of CAT5E type of UTP cable. This is the cable used for peer to peer and local area network. You can see UTP cables in local area networks with RJ-45 couplers in both ends.

The speed of the network performance depends on the network adapter, computer specifications, modem, the number of computers sharing on the network and many other factors but not RJ-45 coupler. In fact, you cannot create a peer to peer and local area

A RJ45-Coupler is a small network cable component which can be used to join two RJ45 cables together to form a longer cable. For example, if you have a two 5m cables you can join them with a RJ45-Coupler to form a 10m cable.

Using a coupler will increase the line resistance of the whole total cable length slightly but I would not expect this to have any noticeable affect on network performance.

Note: If you have speed certified network (probably not at home - but possibly in a high-tech office) then you are not allowed to use RJ45-Couplers.