That router is very old. Does it have enough memory and cpu to load the current images from openwrt. Making it even more confusing there are multiple hardware revisions of that device.
A large restriction on third party firmware is how much it cuts the speed of the router. In your case your internet is so slow I doubt that is a issue.
That does bring up another question. Is you internet connection DSL. Are you going to use the stock router as a modem. I don't think the "new" tplink router has a dsl modem in it. Even if it would that most times means you can not load third party firmware on it. There are license issues with the DSL software so you would actually have to build you own images from the source code and put the drivers in yourself. This is not a simple thing to do.
TL-WR740N is capable for 96...98 Mbps in both directions with stock firmware. Tested personally. If OP claimed internet speed is 3.3/0.5 MBps, he certainly have problem elsewhere. With resource he is using or with his ISP uplink.
To OP: what kind of uplink you have? Do you have other hardware in between like ISP modem or similar?