Slow TC but Modem speed is good

What model TC do you have?? A1xxx from the bottom label or molded rubber please.

How old is it.. at least aprox?

Has this problem always existed or is it new, recent, unable to say?

Have you done a factory reset any time recently to the TC?

The TC is connected to modem via ethernet cable. You asked if I'm connected to TC via ethernet or wireless. So the TC is connected directly to the modem.

Open the airport utility and click edit.. go to the network page. Does it say you are connected DHCP + NAT or off bridge mode.

Is the computer connected to the TC by ethernet? You did not answer that question.. if not ethernet can you please try it.

But you should be able to connect..

modem.. ethernet .. TC... ethernet.. Computer.

Please do it that way.

An A1355 is 3years old now.. and probably does need replacement.

You should do a full factory reset.

The Factory Reset.

Power down the TC.

Hold in reset. and keep holding it in. Be Gentle! It is a tiny surface mount switch with a plastic lever.. Attempting to push it out the other side of the TC will not make the switch turn on better.

(I fix TC and have several reset levers bent over and jammed or broken off).

Power on the TC.. remember with the reset held in.. this needs three arms.. a friend or power switch you can get to with your feet..

Keep holding in reset for about 10sec until the front LED flashes rapidly.

Release reset and wait .. the boot will be slow as it deletes previous setting.. only from router side and pulls out the factory defaults. No files are deleted on the hard disk.. but the name of the TC may change.

Redo the setup after . if it is too slow it needs to be replaced.

Your son maybe connecting at the slow speed all the time (24Mbps in my world is very very good) and so hasn't noticed a slowdown.. in fact most websites are not that fast.. so the difference in speed via the TC and direct to the modem that are concerned about in the normal world of internet usage really is minor..

That is not to say you are wrong to pursue the difference. clearly something is wrong.. the only way you can know is do exact measurements with the PC plugged into the modem directly and then via the TC.. do those measurements and if the TC and getting full modem speed on the PC.. then the finger is pointing back at the Mac.. but it would be strange beyond belief..

When you tested fast on the modem.. for the fault to be the Mac is entirely unlikely!!