Airport Extreme wirelessly bridged to Time Capsule?

I can set the Airport Extreme to extend a wireless network (the 5ghz network offered by the time capsule) and put the network into bridge mode and this works... However performance hasn't improved noticeably from a direct "n" 2.4ghz connection to the Time Capsule from my PS4.

Performance is going to suffer over the wireless "hop" between the Time Capsule and AirPort Extreme. On average, there will be a 40% drop or more in the maximum attainable wireless speeds on the network when you extend using wireless......and this assumes a very good wireless connection between devices.

I don't really want to "extend" the wireless network, I just want to join it

Unfortunately, the AirPort Extreme cannot do this. Ironically, the less expensive AirPort Express will allow this.

I guess I'm worrying that the extension of the wireless network is leading to less than optimal performance as a bridge

Your worry is justified, since you should expect to lose about 50% of the performance when you extend using wireless.

Here is what Apple's support document says about "extending" a network:

In the case of a wirelessly extended network, throughput may be reduced to less than 60 percent of that of a single device. The general rule is to keep the Wi-Fi network as simple as possible. You can accomplish this by using the minimum number of Wi-Fi base stations required to service the physical network area and by using Ethernet wherever possible.

Extending the range of your Wi-Fi network by connecting Wi-Fi base stations together using Ethernet is always the best option, and will provide the best throughput.

Probably not what you want to hear, but the best performance is only attained with a wired Ethernet connection between the AirPort devices, since there is no speed loss over this type of connection.

In the past, the AirPort Exteme could actually be configured to "join" a network.....but the Ethernet ports were not enabled in this type of configuration. The Extreme really did not do anything when it was configured this way.......likely the reason that it cannot now be configured that way.

Only the AirPort Express can "join" a network and allow Ethernet devices to connect. Ironic, since the Express is half the price of an Extreme.

Only Apple can explain things like this.

Also the express only has one lan ethernet port... and only supports "n

If it is an "n" Express, it does support 5 GHz. But, it cannot work with both 2.4 GHz and 5 GHz simultaneously. The newer 2nd Gen Express will work with both 2.4 GHz and 5 GHz simultaneously.

None of the Express models support "ac" wireless, if that is what you are asking about.

If you will connect a 5-port, 8-port, 16-port Ethernet switch to the Express, you will have an many ports as you need.

I wonder if downgrading to the 2.4ghz frequency would improve the efficacy of the wireless bridge

2.4 GHz signals are stronger than 5 GHz signals, and penetrate obstructions like walls and ceilings better than 5 GHz, so that might be worth a try. The downside to 2.4 GHz is that there is more interference on the 2.4 GHz band, simply because there are many more 2.4 GHz networks than 5 GHz networks, and the 2.4 GHz signals are stronger.