Slower combined upstream

I have four ultra-long ADSL2 lines which are IP-bonded together by my Firebrick router and the ISP AA. A couple of months ago, I used to get a real measured combined upstream (TCP payload poss) figure from the speed tester speedtest2.aa.net.uk of 1.56Mbps; but now I only get ~1.2 Mbps.

For the life of me, I cannot see why there is this marked real upstream speed loss because the sync rates have not gone down.

Can anyone suggest what I might be missing ? If the sync rates have not changed significantly why should my effective combined speed be so different ?

This has happened before several times. On one previous occasion, I investigated and came to the conclusion that corruption was causing packet loss which was responsible for TCP retx. But this time I cannot see anything bad anywhere in the stats from the modems which would suggest corruption. The worst line showed ~50 ES in one day recently, with a low CRC value, while the other lines showed much lower daily ES counts. If it would be helpful, I could post the complete stats from all modems.

An other speed-tester test.my/upload gives similar results, so it is not a peculiarity of one particular speed tester.

Live sync rates:
#1: down 2876 kbps, up 525 kbps
#2: down 2762 kbps, up 512 kbps
#3: down 2916 kbps, up 419 kbps
#4: down 2850 kbps, up 496 kbps

The Firebrick is set up as follows: each line to 96.5% of 88.4434% of upstream sync rate. The Firebrick�s current upstream rate limiters' IP PDU tx rates (egress speeds), in-force right now ::
#1: 448076 bps
#2: 436981 bps
#3: 357607 bps
#4: 423325 bps

Total ideal combined rate: 1.665989 Mbps

Edited by CecilWard (Sun 15-Sep-19 02:58:39)