No. 3.5 isn't too bad.
First, make sure you note whether you're getting 3.5 MBps or 3.5 Mbps. The upper- or lower-case B makes a difference of a factor of 8. Big "B" stands for "Byte", little "b" stands for "bit". There are 8 "bits" in each "Byte", hence the factor of 8 thing. Most internet connections and network connections are measured in "Mega-bits per second" while most file transfers are measured in "Mega-Bytes per second". Just noting, the Powerline adapters use the little "b" in their spec, as is normal for network connection speed ratings.
Then there is network overhead: other things the network has to use to keep the connection working right. This can account for quite a bit of your available bandwidth.
Next, while the Powerline adapter may be capable of 500Mbps, what it actual gets is very very dependent on the quality of your home electrical wiring. Unless the house is very new, has incredibly good wiring, and there are no other electrical junctions, circuit breakers, or large appliance in the circuit between the devices, you're highly unlikely to get anywhere near 500Mpbs total throughput on your network.
Take for example my very nice Gigabit (note: it's not GigaBYTE) network here are work. Transferring between two servers right next to each other on the same switch, I'm fortunate to be 100MBps actual throughput in file transfer speed, which is pretty good, because these servers are each connected to 4 Gigabit ports on that switch.
So, all that to say you're probably seeing an OK network speed.