rt-ac87u and asuswrt-merlin

hi,

i am totally a newbie, so i want to apologize in advance for my missing knowledge

so, i have a rt-ac87u on the latest version.
having:
- vpn server running
- dyndns working
- distributed some ips fix to some devices
- wifi running
- and other adjustments

but apparently the standard firmware does not give me all the speed i can get.
see this comment here: https://www.digitec.ch/en/s1/ProductRating/the-flagship-router-for-gigabit-internet-fiber7-144424

my questions are...

rt-ac87u and asuswrt-merlin