The AC66U/R uses the Broadcom BCM4706 running @600 MHz, so it's got a slightly slower processor than the 68U/R and the 68P.
		
		
	 
Wrong, BCM4708/9 ARM (AC68U/P) 
it's not only slightly faster, its ALOT faster than BCM4706 MIPS (N66U/AC66U) specially on tasks using encrytion like OpenVPN, USB transfers, etc.
Clock to clock speeds on ARM arch vs MIPS arch is faster.
You should have performed some benchmarks and see the results fist:
RT-N66U
======
(STOCK SPEED 600MHZ)
admin@RT-N66U-C6C0:/tmp/home/root# openssl speed aes-128-cbc
Doing aes-128 cbc for 3s on 16 size blocks: 1866142 aes-128 cbc's in 2.99s
Doing aes-128 cbc for 3s on 64 size blocks: 502781 aes-128 cbc's in 3.01s
Doing aes-128 cbc for 3s on 256 size blocks: 127832 aes-128 cbc's in 3.00s
Doing aes-128 cbc for 3s on 1024 size blocks: 32130 aes-128 cbc's in 2.99s
Doing aes-128 cbc for 3s on 8192 size blocks: 4015 aes-128 cbc's in 2.99s
OpenSSL 1.0.1k 8 Jan 2015
built on: Wed Jan 14 03:04:32 2015
options:bn(64,32) rc4(ptr,char) des(idx,cisc,2,long) aes(partial) idea(int) blowfish(ptr)
compiler: mipsel-uclibc-gcc -fPIC -DOPENSSL_PIC -DOPENSSL_THREADS -D_REENTRANT -DDSO_DLFCN -DHAVE_DLFCN_H -ffunction-sections -fdata-sections -DL_ENDIAN -DTERMIO -O3 -DLINUX26 -DCONFIG_BCMWL5 -DDEBUG_NOISY -DDEBUG_RCTEST -pipe -DBCMWPA2 -funit-at-a-time -Wno-pointer-sign -mtune=mips32r2 -mips32r2 -DRTCONFIG_NVRAM_64K  -DLINUX_KERNEL_VERSION=132630 -DOPENSSL_SMALL_FOOTPRINT -fomit-frame-pointer -Wall -DSHA1_ASM -DSHA256_ASM -DAES_ASM
The 'numbers' are in 1000s of bytes per second processed.
type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes
aes-128 cbc       9986.04k    10690.36k    10908.33k    11003.72k    11000.29k
(OVERCLOCKED TO 660MHZ) => YOU CANT OVERCLOCK IT MORE THAN THAT.
admin@RT-N66U-C6C0:/tmp/home/root# openssl speed aes-128-cbc
Doing aes-128 cbc for 3s on 16 size blocks: 2084745 aes-128 cbc's in 3.02s
Doing aes-128 cbc for 3s on 64 size blocks: 558802 aes-128 cbc's in 3.02s
Doing aes-128 cbc for 3s on 256 size blocks: 141141 aes-128 cbc's in 3.00s
Doing aes-128 cbc for 3s on 1024 size blocks: 35919 aes-128 cbc's in 3.02s
Doing aes-128 cbc for 3s on 8192 size blocks: 4488 aes-128 cbc's in 3.02s
OpenSSL 1.0.1k 8 Jan 2015
built on: Wed Jan 14 03:04:32 2015
options:bn(64,32) rc4(ptr,char) des(idx,cisc,2,long) aes(partial) idea(int) blowfish(ptr)
compiler: mipsel-uclibc-gcc -fPIC -DOPENSSL_PIC -DOPENSSL_THREADS -D_REENTRANT -DDSO_DLFCN -DHAVE_DLFCN_H -ffunction-sections -fdata-sections -DL_ENDIAN -DTERMIO -O3 -DLINUX26 -DCONFIG_BCMWL5 -DDEBUG_NOISY -DDEBUG_RCTEST -pipe -DBCMWPA2 -funit-at-a-time -Wno-pointer-sign -mtune=mips32r2 -mips32r2 -DRTCONFIG_NVRAM_64K  -DLINUX_KERNEL_VERSION=132630 -DOPENSSL_SMALL_FOOTPRINT -fomit-frame-pointer -Wall -DSHA1_ASM -DSHA256_ASM -DAES_ASM
The 'numbers' are in 1000s of bytes per second processed.
type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes
aes-128 cbc      11045.01k    11842.16k    12044.03k    12179.16k    12174.07k
RT-AC68U
========
(STOCK SPEED 800MHZ)
admin@RT-AC68U-F3B0:/tmp/home/root# openssl speed aes-128-cbc
Doing aes-128 cbc for 3s on 16 size blocks: 4730852 aes-128 cbc's in 2.94s
Doing aes-128 cbc for 3s on 64 size blocks: 1308811 aes-128 cbc's in 3.00s
Doing aes-128 cbc for 3s on 256 size blocks: 340155 aes-128 cbc's in 3.00s
Doing aes-128 cbc for 3s on 1024 size blocks: 85905 aes-128 cbc's in 3.00s
Doing aes-128 cbc for 3s on 8192 size blocks: 10783 aes-128 cbc's in 3.00s
OpenSSL 1.0.1k 8 Jan 2015
built on: Wed Jan 14 05:00:52 2015
options:bn(64,32) rc4(ptr,char) des(idx,cisc,16,long) aes(partial) idea(int) blowfish(ptr)
compiler: arm-brcm-linux-uclibcgnueabi-gcc -fPIC -DOPENSSL_PIC -DOPENSSL_THREADS -D_REENTRANT -DDSO_DLFCN -DHAVE_DLFCN_H -ffunction-sections -fdata-sections -DTERMIO -O3 -Wall -DOPENSSL_BN_ASM_MONT -DOPENSSL_BN_ASM_GF2m -DSHA1_ASM -DSHA256_ASM -DSHA512_ASM -DAES_ASM -DGHASH_ASM
The 'numbers' are in 1000s of bytes per second processed.
type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes
aes-128 cbc      25746.13k    27921.30k    29026.56k    29322.24k    29444.78k
(OVERCLOCKED TO 1.2GHZ) => YOU CAN OVERCLOCK IT MORE THAN THAT.
admin@RT-AC68U-F3B0:# openssl speed aes-128-cbc
Doing aes-128 cbc for 3s on 16 size blocks: 8324959 aes-128 cbc's in 2.98s
Doing aes-128 cbc for 3s on 64 size blocks: 2281026 aes-128 cbc's in 2.98s
Doing aes-128 cbc for 3s on 256 size blocks: 586216 aes-128 cbc's in 2.97s
Doing aes-128 cbc for 3s on 1024 size blocks: 146351 aes-128 cbc's in 2.92s
Doing aes-128 cbc for 3s on 8192 size blocks: 18532 aes-128 cbc's in 2.94s
OpenSSL 1.0.1k 8 Jan 2015
built on: Wed Jan 14 05:00:52 2015
options:bn(64,32) rc4(ptr,char) des(idx,cisc,16,long) aes(partial) idea(int) blowfish(ptr)
compiler: -I. -I.. -I../include  -fPIC -DOPENSSL_PIC -DOPENSSL_THREADS -D_REENTRANT -DDSO_DLFCN -DHAVE_DLFCN_H -ffunction-sections -fdata-sections -DTERMIO -O3 -Wall -DOPENSSL_BN_ASM_MONT -DOPENSSL_BN_ASM_GF2m -DSHA1_ASM -DSHA256_ASM -DSHA512_ASM -DAES_ASM -DGHASH_ASM
The 'numbers' are in 1000s of bytes per second processed.
type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes
aes-128 cbc      44697.77k    48988.48k    50529.06k    51323.09k    51637.46k