The spring you refer to is probably deflection. Whenever you span joists between their end supports the center of the span will deflect some minor distance when it is flexed by weight like someone walking on the deck. How much deflection can you tolerate so the floor does not feel springy ? Many deck codes call for a 40psf structure which will bounce when you walk on it. A rigid floor has 0 deflection but this costs in material used.
Most timber selection building charts for decks will list the nominal deflection for a given joist size. if you use 6x6 or 8x8 material for your posts spaced 5' (matches the 20' long side nicely) with the same 4x8 for the band joists (around the edge)then you will have a pretty ridid system to hang your intermediate joists from.When you put the decking on run it at 45 degree angle across the joists and the whole structure will be more rigid. Use deck screws (no nails) and a bead of liquid nails on the joist tops for a long lasting bit of work which won't squeek. The deck behind my house has two differenct support systems - the strong floor uses 6x6 posts on 5' centers with the 2x10 joists making a sandwich of the posts attachment with thru bolts. The 2x5 decking is angled so the net floor has zero deflection and tests in excess of 200lbs psf load.
choose carefully before you build.