OpenWRT 18.06 for TP-Link Archer C7 AC 1750 v5 is an ar71xx target which is on an older kernel (4.9) and does not support fast path.
Highlights of this release since the previous LEDE 17.01 and OpenWrt Chaos Calmer 15.05 versions are:
Linux kernel updated to version either 4.9.111 or 4.14.52, depending on the architecture
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Updated Linux kernel:
Kernel 4.9.111 for ar7, ar71xx, arc770, at91, brcm2708, brcm63xx, ixp4xx, lantiq, layerscape, mpc85xx, orion, rb532 and uml
Kernel 4.14.52 for apm821xx, archs38, armvirt, ath25, bcm53xx, brcm47xx, cns3xxx, gemini, imx6, ipq40xx, ipq806x, kirkwood, malta, mediatek, mvebu, mxs, octeon, octeontx, omap, oxnas, pistachio, ramips, sunxi and x86
The soon to be released 19.07 will be ath79 target which is on kernel 4.14 an supports fast path.
Or you can install a snapshot (ath79 target) today.
ToH Archer C7 v5