The device looks like it has a 400MHz cpu so it most likely reaches the limit of throughput rate. You can verify that by running a top command while performing the speedtest. Check the cpu utilization on the second line and pay attention to the idle % value.
Well, that's how it is... You can do some tweaks which might increase performance slightly but ~140-150Mbit is pretty much the limit for the AR9132 CPU and that's excluding PPPoE.
with SFE patch it can do >400 mbps via pppoe and ~700mbps ipoe (simple nat)
if you can't/don't want to patch it you can try dd-wrt that has SFE included
SFE wasn't accepted by the openwrt developers.
they implemented a different technology - flow offload - available only for ath79 targets.
your router wasn't included yet in this ath79
The flow-offloading infrastructure has been merged into the mainline kernel (not just OpenWrt) with kernel 4.16 (and was backported to OpenWrt's 4.14 kernel), it's available to all architectures with a current enough kernel. Maintenance for this new feature is covered by the mainline netdev developers and doesn't require the OpenWrt developers to maintain it (while SFE is an out-of-tree implementation, which would have needed constant attention by OpenWrt developers). Furthermore the flow-offloading feature provides a unified interface for software based offloading and hardware acceleration (currently only for mt7621, but AR8327N/ QCA8337N and lantiq (potentially even the ipq806x NPU/ NSS cores) would be other potential candidates).