q is good with bulk operations on compact arrays; these are cache-friendly and the interpreter can utilize cache-level parallelism. and with q it's convenient to go from idea -> MVP in short time. it's a high-level language with functional features so expressing algos and complex logic is natural.

but it's interpreted and optimized for array ops. so really latency-critical (e.g. high-freq trading) or highly scalar logic will be done with C++. the trade-off is convenience of development.