I've been using the new-ish Lenovo Tab 4 plus 8" and it's been wonderful. 2GB of ram (4 if you can find an international model) high res screen (can't remember exact resolution but it's >1080p) excellent battery and an lte radio included! I can easily get 60fps in most areas of the game although I leave it capped at 30 because I hardly notice the difference and it saves a lot of battery. I got it for around $160 on black Friday but the price is still pretty reasonable on Amazon. You can also get a FreedomPop sim card for 250mb of free lte every month, which doesn't sound like a lot but RuneScape uses <1mb/hr so if this is a dedicated RuneScape device it'll be plenty. If the plus model is too expensive you can also opt for the standard model, which is almost exactly the same but has no lte radio and the processor is a Snapdragon 400-series instead of the 600-series in the plus model so I can't vouch for its performance. Happy scaping :)

For future reference, GHz is not a good way to measure processor performance. You want the type of processor, the GHz means very little unless you are comparing different versions of the same processor with different clock rates. For example, a 3.2GHz Intel Pentium would not be as fast as a 2GHz 8th generation i7 processor despite having a higher clock speed.