make a settings menu, and give the player the possibility to reduce quality in a detailed and a simple mode. in simple mode you could set low mid and high quality groups of settings, in case of low you can disable shadows, but imo it is quite an ugly solution.

you can play with lod and shader complexity a lot (simple shader for far lod).

if you would show some screenshots it would be useful. 30 fps is not bad on a budget pc.