Short Preamble
If you have 802.11b clients in your network try enabling this, if they have problems connecting or with performance then leave it disabled. Preamble is at the head or front of the PLCP which devices need in order to start transferring data. The long preamble ensure compatibility with legacy 802.11b devices but can slightly reduce throughput at high data rates. The support for short preamble which is reducing the header's size by 50%, down to 9 bytes, is optional for 802.11b. 802.11g and newer all support short preamble, so if you do not have 802.11b devices in your network leave this enabled at all times.

Protection Mode
Recommended Setting: None for AP modes, CTS or RTS/CTS for client modes

This setting controls whether the request to send/clear to send 802.11 optional protection mechanism is enabled or disabled. When enabled, an RTS/CTS handshake must be completed before data can be transmitted from clients. Helpful in noisy &/or busy environments, it ensures all clients take turns communicating with the AP, if disabled, packet collisions may occur which causes a drop in throughput & increase in latency due to retransmission overhead. RTS/CTS also helps negate the hidden node problem which occurs when 2 or more clients can each see the AP & vice versa, but the clients can't see eachother, this example is also good to say why RTS/CTS on an AP has no use, since from the AP's point of view, it can already see all connected clients, or they wouldn't be connected in the first place. CTS only is "CTS-to-self" which has less overhead, but is less effective in mitigating the hidden node issue, only other clients within range of the client using CTS only, will hear & honor it while RTS/CTS is the "full option" that gets passed through the AP to all clients, even if the AP has RTS/CTS disabled since RTS/CTS on the AP only applies to when the AP wants to transmit.
RTS/CTS is a setting to experiment with especially on the client mode interface of the router if you are connecting a router to another router, or if you have high error rate or high noise floor (-90 noise is good, -60 is bad) & all other options have failed. Most users should leave this set to the recommended setting above for max performance because the protection mechanism is only enabled automatically when needed, if its off when its needed, your wireless performance can plummet with errors, disconnects & low throughput, & if its no longer needed its turned off automatically on the fly.

Example of an ideal configuration for a client mode router connected to a host router:
Client router with RTS/CTS on (client/client bridge/WDS station) ---> Host router with RTS/CTS off (AP/WDS AP)

Short GI

Recommended Setting: Enable

The standard guard interval used in 802.11 OFDM is 0.8 μs, to increase data rate 802.11n added optional support for a shorter 0.4 μs guard interval which provides about a 10% increase in data rate. The shorter guard interval could (but usually doesn't) result in a higher packet error rate if timing synchronization between the transmitter and receiver is not precise. To reduce complexity, short guard interval is only implemented as a final rate adaptation step when the device is running at its highest data rate such as 72 Mbps, 144 Mbps, 300 Mbps etc, this is by design & not changeable.
Older routers & devices with Atheros AR92XX radios or older only support short GI on HT40 & not HT20, so max HT20 rates are 65 Mbps/130 Mbps/195 Mbps (1x1/2x2/3x3) instead of 72 Mbps/144 Mbps/217 Mbps respectively. Some modern devices such as the Playstation 4 do not like the lack of short GI, & have strange performance problems, sometimes completely crippling the entire network's performance. But the issue may also be related to hardware bugs in the AR92XX chipset, mileage may vary.