Sorry missed this reply. Only other thing I can think of is maybe change to another channel that is clearer based on your surroundings. Also if others are are your network (kids esp) make sure they are not running some sort of file sharing service or something sucking up your bandwidth. I have run into that several times with folks having issues with speed.
Thanks for the suggestion. I have now installed merlin firmware 384.6 and the speed is back to normal. What is notice is the wifi link speed on merlin-fork can only reach about 173 MB on my device and that should be why the download speed is massively limited. I'm not sure if adjusting the tx power on the wifi setting would have helped on the merlin-fork firmware. I'm happy at the stable speed for the moment, only concern is the lack of security patches.