AirPort Time Capsule 3rd generation

when I go through it AirPort Time Capsule 3rd generation I get 270Mbps

Remember that the 3rd Gen Time Capsule was designed in the 2008-2009 timeframe, so this is about the best that you can do with this product Why? The WiFi chipset that is used in this version of the Time Capsule is rated at about 300 Mbps, and there will always be some loss of speed in the connection process.

If you had a 4th Gen Time Capsule, you could expect a top speed of about 450 Mbps.

If you have the 802.11ac version of the Time Capsule, which is unofficially known as the 5th Gen Time Capsule, you might be able to attain speeds of up to 1300 Mbps if your computer or other devices are capable of that level of speed.

Since your Time Capsule was discontinued in the summer of 2011, so your product is at least 10+ years old, and as much as 12 depending on when the product was placed into service.

Frankly, it is amazing that your Time Capsule will still operate at all, much less perform up to specification levels. Pat yourself on the back for your very very good luck with the product. Mine lasted less than 5 years.

Are you testing the Gen3 TC by ethernet or wifi?

As others have pointed out the wifi is older N wireless and on its best day will not get better than your 270mbps actual throughput.. in fact that is very good.

Even AC wireless will only just achieve 500Mbps throughout on a Gen5 TC.. due to lack of processor speed and wifi limitations.

If you are testing via ethernet I would say you should achieve 400Mbps or perhaps a bit more or less.

If your ISP is compatible with using IPv6 that will improve by a country mile.

A gen3 TC is well past its use by date anyway. If you wish to continue using it for backups.. plug it by ethernet to your other router in bridge mode.. and turn off its wireless... you can backup to it still although this is a bit tricky and if you need further help to make it reliable post back.

About 300 mbps is the theoretical limit of WI-Fi n, which is what the AirPort Time Capsule (3rd generation) has.