Best way to reduce lag on LAN?

The best solution if you need a switch is a non smart or non managed switch to use with any router that already has QoS and traffic management features. The smart or managed switches are generally needed at different configurations for use in other networks that need either specific traffic routing to other swtiches or in some cases VLAN configurations. Usually HW routers have most of these features already so by using a smart or managed switch with a HW router will be kind of redundent or over use of certain features and can cause problems. So using a non-managed switch is preferred with HW routers. If you have a managed or smart switch used with a HW router, then be sure to disable all the traffoc management features on the swtich and let the HW router do all that.

I've been using a 24 port non managed switch for years with HW routers with out any issues. Let the HW router do the job and be the main point of doing network management and handling. Use swithces for adding additional ports for devices that you need to add to the routers network since most routers are only 4 port capable. Nothing wrong in adding a non-managed switch.