Ath9k & separate adapter card 802.11n 2.4 MIMO speed

Who are using openwrt on device with separate pci Atheros card (not a SoC!) and ath9k driver in MIMO mode at 2.4 802.11n -- which maximum speed do you get?

I have Unielec 7621-06, MTK7621AT chipset, 512 RAM, 64 NAND,and Atheros AR9287 mini pci-e card.

In case of enable hwcrypt I have up to 65 mbit/s but connection losts very often for any client (Androind, Windows, TV).

In case of disablng hwcrypt connection became much more stable but speed does not increase 25-30 mbit/s

Openwrt 19.07.3