how to start troubleshooting slow wireless connection (iPhone<->2011UAS-2HnD)

RouterOS Wireless Networking
1

I experienced low speed connection between my 2011UAS-2HnD and iPhone 6)
my iPhone 6 connects at lower speed compare to my lenovo x250
so my x250 connects @150Mbps (TX/RX signal ~-61 dBm)
but my iPhone 6 would not go more than 72Mbps with TX/RX signal ~ -49 dBm

any explanation?
also wonder to know if there is any document which can help to dig deeper wifi slow connection and slow transfer rate

2

Your router has two radio chains.

I do not have an iPhone. My Samsung Galaxy S4 only has one radio chain. If your iPhone is like my Samsung, that is why your iPhone shows half the speed of your laptop which has two (or more) radio chains.