Link aggregation can kind of be thought of as a highway. Adding additional lanes to the highway will increase the total number cars that can be on the highway, and reduce congestion. However the speed limit of the highway stays the same.  Your findings are correct, and is the expected behavior of a LAG.
switch is 2 N4032 with 1 40g stacking cable. jumbo frames are not yet enabled. two dell r720s with 10g ethernet lacp across both switches. one running ubuntu 16.04, the other debian jessie with kernel 4.4.6. iperf3 across them shows 8.9 Gbits/sec, which is what i would expect on a single port. does the lag across switches slow it down to the speed of one? 
speed of lacp across switches