The algorithm used by WinRAR to hash the password that protects the encryption key - PBKDF2-HMAC-SHA256:
6. Changes in RAR 5.0 encryption algorithm:
a) encryption algorithm is changed from AES-128 to AES-256 in CBC mode.
Key derivation function is based on PBKDF2 using HMAC-SHA256;
... is designed to be slow, and specifically to be resistant to high-speed offline attacks ... though CPU will indeed be slower:
$ hashcat -w 4 -O -b -D 1 -m 10900
hashcat (v4.1.0) starting in benchmark mode...
* Device #7: AMD FX(tm)-8350 Eight-Core Processor, 8034/32139 MB allocatable, 8MCU
Hashmode: 10900 - PBKDF2-HMAC-SHA256 (Iterations: 999)
Speed.Dev.#7.....: 21213 H/s (370.38ms)
... than GPU:
$ hashcat -w 4 -O -b -D 2 -m 10900
hashcat (v4.1.0) starting in benchmark mode...
* Device #1: GeForce GTX 1080, 2028/8113 MB allocatable, 20MCU
* Device #2: GeForce GTX 1080, 2028/8114 MB allocatable, 20MCU
* Device #3: GeForce GTX 1080, 2028/8114 MB allocatable, 20MCU
* Device #4: GeForce GTX 1080, 2028/8114 MB allocatable, 20MCU
* Device #5: GeForce GTX 1080, 2028/8114 MB allocatable, 20MCU
* Device #6: GeForce GTX 1080, 2028/8114 MB allocatable, 20MCU
Hashmode: 10900 - PBKDF2-HMAC-SHA256 (Iterations: 999)
Speed.Dev.#1.....: 1222.0 kH/s (478.00ms)
Speed.Dev.#2.....: 1204.6 kH/s (484.15ms)
Speed.Dev.#3.....: 1213.5 kH/s (481.70ms)
Speed.Dev.#4.....: 1210.2 kH/s (482.47ms)
Speed.Dev.#5.....: 1220.8 kH/s (477.90ms)
Speed.Dev.#6.....: 1214.7 kH/s (480.94ms)
Speed.Dev.#*.....: 7285.8 kH/s
These benchmarks assume ideal conditions. Real-world speed may be on the order of half of this speed or less, depending on the type of attack.
As long as the rate at which candidate passwords are generated is high enough, and the native hashes-per-second rate possible due to the algorithm itself and its work factors (iterations, etc.) is slow enough, the length of the candidate passwords does not impact the guess rate.