Assuming that they are on the same technology (which
you do not state) there's just the extra read-path load
requiring additional slew time on write, to get to the
positive feedback tipping point (plus margin).
But SRAM technology node matters and comparing cell
topologies across foundry or flow is pointless.
8T is often applied as a radiation effects mitigation and
even within a technology, you might see 8T cells use
high-VT devices for total dose effects while 6T commercial
designs might use LVT for speed (unless they are after
low power foremost, then RVT).
Don't ask for simple answers to questions like this. Do
the reading. Details are everything.