Characteristic drop in upload using traffic shaper and limiters

[Tillebeck]

I have attached a dump of traffic graphs.
After adding traffic shaper and limiters with mask I get a very characterisitic grapgh for each speedtest.

Dooing a speedtest from LAN shows a strange graph for the upload.

Download raise fast and is fairly stable as expected.
Upload start at a decent speed and then drops as seen on the attached image.

The limiter is set to 55Mbit down and 95Mbit up. So the upload never reaches it potential. It starts out at half or a third of the max speed and then it drops over the next 30 seconds. i can reproduce this as many times as I want.

Just from looking at the characteristic graph. Can anyone tell what could cause this behaviour?