TM-AC1900 WAN slow down and CPU loading increase on newest Merlin ,and no way back?

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New Around Here
This is NOT a tech post or asking help for tech issue, and I am not trying to discuss any "flashing/re-branding" here, but just to raise some concern/awareness about Merlin FW (maybe origin Asuswrt FW)
I will use AC68U below for original ASUS AC68U, and AC1900 for flashed/re-branded AC68-variant from T-mobile TM-AC1900 . I will use Asuswrt for original Asus FW, and Merlin for Asuswrt-Merlin branch/fork FW.
I will mask some details below to remain anonymous.

I have been using original AC68U v1 for several years with Merlin, always on the latest version and it is all good.
I bought two AC1900 several years back hoping to build a MESH with wired back-haul, but did not have time to get it down. Both of the AC1900 were immediately flashed and re-branded, and they stayed at Merlin Ver 380.xx from then.

Recently, I decided to re-deploy the AC68U to some other location and I was planning to use one AC1900 at my own place. I backed up the settings from Ac68U, and restored to AC1900 after I upgraded the Merlin on AC1900 to the newest Ver 386.xx.
However, connected to FIOS ONT, I find the WAN speed can not get over 100Mbps while I am on FIOS 300Mbps plan. I tried the speedtest with Wifi connection from laptop/desktop, and wired connection from laptop and from built-in Entware, all the same.
I then connected laptop directly to the ONT, and I can confirm the speed from ONT is 300Mbps. I switched the AC68U back, and I found the WAN speed went back to 300Mbps. Then, I realized there must be something wrong with the AC1900.

I have tried several methods below (both with my backed setting AND default setting), hoping to get back 300Mbps on the AC1900, not gain.
1) I flashed the newest Asuswrt FW (the 140MB mega one), the aged-newest Asuswrt FW, and old Asuswrt.
2) I flashed the second newest Merlin and Ver 384.xx Merlin
3) I flashed V380.xx Merlin (exactly same version as the other untouched AC1900 )

However, I connected the other untouched AC1900 to the ONT, it could give me 300Mbps WAN speed.

I repeated the comparison above again to double check, and for the second time, I not only paid attention to the WAN speed, but the CPU loading percentage.

I can say that once the new Merlin (maybe new Asuswrt ???) was flashed onto the AC1900, even a FW (Merlin or Asuswrt) downgrade was performed, the AC1900 would still have substantial reduction in WAN SPEED and increase in CPU loading percentage (especially when basic Entware environment is installed without any add-on)

I found on another forum (sorry, I can not disclose where) that someone experiencing a similar issue as mine, while that guy MANAGED to get a full speed WAN back by flashing a T-Mobile FW to AC1900. The same guy had another AC1900 untouched on Asuswrt FW, and it can get full WAN speed.

This reminds me of last time, a precision surgery targeted on MTD and forced roll-back, now it is more sophisticated.......


Asuswrt-Merlin dev
You are running my firmware on a non-supported model. There is nothing more for us to tell you, sorry.
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