30Amp mosfets have large internal geometries. So they tend to have alot of capacitance as a result. In order to switch them "fast" you will also nee d a suitable gate drive circuit. First things first, you need to pick a power mosfet to use that will be a suitable replacement. Consider also paralleling several smaller ones.
We can get a better idea of how to replace the 30A relay if you can provide a schematic of how & where the relay is currently being used.
In the most basic application, your tie your 30A load to the drain(s) of the Power FETS and drive the gate w.r.t. the source with something like 10-15V but if all you have is 5V in your system, there are ways to drive the fets with that.. Again, we need some more detail & schematic if you want "detailed" help.