it's not unusual for routers (or anything else for that matter) to under-perform on various benchmarks. as with anything, the right data can be selected to show that products meet their advertised specs, even if various other benchmarks don't agree. but another thing to bear in mind is that benchmark results don't always represent the experience you will have in real use case scenarios. the advertised benchmarks of ssd's are a good example of that.
if you notice this difference in real-world uses and really want to do something about it, you're probably going to want a new router. it looks like the wndr3700v3 was released in 2011, so it's probably time. it sounds like you've already done all of the things that could be suggested (firmware, drivers, etc). one last thing you might try is cleaning the router out if it's dusty, and making sure that there is reasonable airflow for cooling.
i like smallnetbuilder.com for consumer network hardware benchmarks. unfortunately they don't seem to have tested any versions of the wndr3700. most decent new routers should have nearly perfect wired throughput (~940Mbps), but you can check out the benchmarked performance on smallnetbuilders' router charts and/or read their reviews of a router you are considering.
https://www.smallnetbuilder.com/tool...-to-lan-tcp/35
