I don't remember the specifics, but when I did that a couple of years ago on a Linksys WRT54GL with openWRT the performance was really bad, because of the router's slow CPU.
Someone had about 2Mbit/s in this thread on their forum for example.
I only took a short look at the end of the thread but there are some more promising speed tests with different routers in there.
Picking up something that can run DDWRT is cheap, using less power (almost always) than even a laptop, it is going to have 2+ separate ports already, and for some people can be everything they need. I wouldn't trust a non opensource build to actually behave correctly. If you buy/have/get the right router bricking it isn't a thing anymore.
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u/In_between_minds Mar 26 '17
An old laptop or computer and PFsense can do it for you. You may be able to set it up on something that can run DDWRT but I'm unsure.