Make all heap objects store a reference to their class and inline class lookup.
This is redundant, but significantly optimizes method dispatch:

binary_trees - wren           ..........  3347  0.30s  110.30% relative to baseline
delta_blue - wren             ..........  7161  0.14s  110.39% relative to baseline
fib - wren                    ..........  2445  0.41s  112.78% relative to baseline
for - wren                    ..........  8600  0.12s  143.00% relative to baseline
method_call - wren            ..........  5112  0.20s  114.15% relative to baseline