How feasible is a preimage attack or speedups to brute force a substring of MD5?
Right now, the cheapest known algorithms for (partial) preimage search are generic algorithms, costing ${\sim}2^b/t$ to compute a preimage for any of $t$ target $b$-bit strings in time ${\sim}2^b/t^3$ if parallelized $t^2$ ways—here $b = 128 - 24 - 4 - 20 = 80$.

The Sasaki–Aoki algorithm, while perhaps theoretically interesting, costs about a petabyte of memory, when at much lower cost you could power 32 CPUs (or just MD5 circuits) instead and get an answer faster with a naive generic search in parallel.