diff --git a/sys/dev/rtwn/rtl8812a/r12a_var.h b/sys/dev/rtwn/rtl8812a/r12a_var.h
--- a/sys/dev/rtwn/rtl8812a/r12a_var.h
+++ b/sys/dev/rtwn/rtl8812a/r12a_var.h
@@ -99,6 +99,7 @@
 	int		ac_usb_dma_size;
 	int		ac_usb_dma_time;
 	int		ampdu_max_time;
+	int		ampdu_max_size;
 };
 #define	R12A_SOFTC(_sc)	((struct r12a_softc *)((_sc)->sc_priv))
 
diff --git a/sys/dev/rtwn/rtl8812a/usb/r12au_attach.c b/sys/dev/rtwn/rtl8812a/usb/r12au_attach.c
--- a/sys/dev/rtwn/rtl8812a/usb/r12au_attach.c
+++ b/sys/dev/rtwn/rtl8812a/usb/r12au_attach.c
@@ -141,6 +141,7 @@
 	rs->rs_iq_calib_sw		= r12a_iq_calib_sw;
 
 	rs->ampdu_max_time		= 0x70;
+	rs->ampdu_max_size		= 0x1ffff; /* 128k */
 
 	sc->sc_priv = rs;
 }
diff --git a/sys/dev/rtwn/rtl8812a/usb/r12au_init.c b/sys/dev/rtwn/rtl8812a/usb/r12au_init.c
--- a/sys/dev/rtwn/rtl8812a/usb/r12au_init.c
+++ b/sys/dev/rtwn/rtl8812a/usb/r12au_init.c
@@ -142,7 +142,17 @@
 
 	/* Setup AMPDU aggregation. */
 	rtwn_write_1(sc, R12A_AMPDU_MAX_TIME, rs->ampdu_max_time);
-	rtwn_write_4(sc, R12A_AMPDU_MAX_LENGTH, 0xffffffff);
+	/*
+	 * Note: The vendor driver (hal/rtl8812a_hal_init.c:SetHwReg8812A())
+	 * also sets bit 31.
+	 */
+	/*
+	 * TODO: this should be limited to the peer in STA mode,
+	 * and perhaps the minimum A-MPDU of all VAPs/peers in
+	 * multi-STA / other operating modes.
+	 */
+	rtwn_write_4(sc, R12A_AMPDU_MAX_LENGTH,
+	    rs->ampdu_max_size | (1<<31));
 
 	/* 80 MHz clock (again?) */
 	rtwn_write_1(sc, R92C_USTIME_TSF, 0x50);
diff --git a/sys/dev/rtwn/rtl8821a/usb/r21au_attach.c b/sys/dev/rtwn/rtl8821a/usb/r21au_attach.c
--- a/sys/dev/rtwn/rtl8821a/usb/r21au_attach.c
+++ b/sys/dev/rtwn/rtl8821a/usb/r21au_attach.c
@@ -141,6 +141,7 @@
 	rs->rs_iq_calib_sw		= r21a_iq_calib_sw;
 
 	rs->ampdu_max_time		= 0x5e;
+	rs->ampdu_max_size		= 0xffff; /* 64k */
 
 	rs->ac_usb_dma_size		= 0x01;
 	rs->ac_usb_dma_time		= 0x10;