Strange speedtest graph

I am seeing my TBB speedtest graphs showing a slow buildup. It seems to take 25-30% of the test to build up to max. level. Clicking on the result shows the graph.


Speeds are not the problem, it is the strange buildup. A couple of months ago the buildup on download had a sharp climb like the upload.

BQM is also showing great variation in latency during the day
My Broadband Ping

Has anyone thoughts as to what is happening?

Regards
David

Are you sure nothing is limiting the RWIN size on the device you are testing with.

If sure that this not the reason for slow rise, it may be the providers network. Try forcing IPv4 to see if any different, but need to also check routing as ipv4 and ipv6 may take a different route.

From speedguide.net:

MTU = 1492
MTU is optimized for PPoE DSL broadband. If not, consider raising MTU to 1500 for optimal throughput.
MSS = 1452
MSS is optimized for PPPoE DSL broadband. If not, consider raising your MTU value.
Default TCP Receive Window (RWIN) = 29312
RWIN Scaling (RFC1323) = 7 bits (scale factor: 2^7=128)
Unscaled TCP Receive Window = 229

ipv6 and ipv4 show similar graphs

Zen assure me that there is nothing wrong with their network and my line, and say that the graph is normal

Taking a few seconds to reach max speed is not normal, if the ISP thinks that is normal perhaps time to look elsewhere once you've tried the PC on a different connection and proved its the device/OS