Drop in Speed between airport express and express

Hi all

I am quite new to this apple networking and got an issue to ask.

This is my environment. I am using 10mbps adsl. The encryption is WPA-2 Personal. All devices are using static IP. Airport extreme is 5th Generation and Airport express is 2nd generation with latest firmware.

I am connecting from my modem to airport extreme (using 2.4GHz on Channel 8) and to airport express (EX2). The location is about 70m away with 1 wall in between. I am able to achieve around 9-10mbps.

From Ex2, I join it using "Join to wireless network" and connect with physical straight cable LAN to WAN of another airport express (EX3). EX2 is on automatic channel for both 2.4GHz and 5 GHz (automatic as I cant change them). I set EX3 as "extend wireless network and is (using 2.4GHz on Channel 8).

I use airport express (EX1) and place it about 50m away. There is one server room with many networking devices in it. EX1 is on automatic channel for both 2.4GHz and 5 GHz (automatic as I cant change them).

Now I did a speed test on EX1 location and can only achieve 1.5-2.5mbps. I did a wifi detection test and there are many networking devices on channel 6 and 11 with few on channel 1-5.

I did tried to change manually for the 2.4GHz channels from 1-13 and 5GHz channels. The test speed results remain the same 1.5-2.5mbps at EX1.

Now my question is if I extend from airport express (EX3) to airport express (EX1), will it cause a drop n speed at EX1? Or was it due to the server room that causes the poor performance? I do not have access to the server room nor those devices in the room.