try to set the tdma-period-size to 2 and check if the speed got stable.
Also please upgrade the RouterOS to v6.18
Can you explain why tdma-period-size should be 2? Not so much the number but the explanation why interests my.
I have 95% of my network (+30 routers in either backhaul or AP (wireless) mode) and they all work fine it seems with fastpath and hardware queue on the interfaces.
I only have one rb800 which has one AP radio with 37 clients in a mixed range from 2km up to 8km and here the wireless fp package completely destroys the AP-network.
Since NV2 with its tdma period size and the hardware queue and the fastpath setting all seem to have a relation with each other I am trying to get more in-depth information on why I run into problems with rb800
[And maybe the problem in the rb800 only surfaces because it has by far the most clients connected to one AP radio, that are also have a relative high distance range for these clients that also all are working in single chain mode protocol N with 20Mhz channel width.
I played with several settings, like the tdma-period-size, the channel width (but run into interferences with other AP), fixed or default conn. rates+MCS rates but the only thing to bring the network back to normal again is disabling wireless-fp package to use the normal one again. I made a ticket for this issue but so far no answer... (couple of days now..)