Just a guess: you might have run into the hidden node problem. https://en.wikipedia.org/wiki/Hidden_node_problem
Try to avoid this by turning on MAXtream.

Re speed: for 20 MHz channel width max. speed of index MCS15 is 130/144.4 Mbps. Only for 40 MHz channel width MCS15 equals 270/300 Mbps. But channel width 40 MHz requires 8 WiFi channels in the 2.4 GHz frequency band, so one can achieve this usually only in desert islands, not in areas with a dense population.

ggorbalan wrote

If you set on the AP 20 MHZ it should do the same for the "Repeater" on 20 MHZ and not do 40..... it should replicate the 1st signal, on the same channel and width

First of all you miss the point: AP client router mode has nothing to do with "repeating", in fact it is not repeating at all, but routing. And no, it does not have the same channel width for the client to CPE link as for the CPE to CPE link, but yes, it needs to use the same channel (what is exactly the cause for a potential hidden node problem).

But even for a CPE in repeater mode it shows the same behavior as in AP client router mode regarding the wireless settings:

You have to differentiate between capabilities of the AP and actual negotiated operation mode. If you set your AP to 20 MHz channel width, the client still can be set at 40 MHz. What the client "changes" is just the actual operation mode to 20 MHz channel width for this link if it connects to the AP, b/c it negotiates this mode with the AP. This does in no way change the client's settings, so if you set the root AP back to 40 MHz, the client would immediately start using this mode.

Proof: A CPE in AP client router mode with channel width up to 40 MHz (max. TX rate: 300 Mbps) connected to a CPE in AP mode with channel width 20 MHz (max. TX rate: 130/144.4 Mbps) use dynamically negotiated 130/144.4 Mbps:






Again, I bet: any unexpected degradation in speed on the CPE1/CPE2 link when transmitting data from client to CPE3 connected to CPE2 is caused by the hidden node problem.