droplet net speed slow, any idea what i am doing wrong?

i had a droplet in the SF region for a few days, it was doing well. download speed is 100K-400K/s, the fastest time is ove 1M/s. but one 2-3 days ago, it suddenly slowed down. at the worst, i had trouble using putty to login. the speed becamse 10K-20K/s. i destroyed it and recreated it to no avail. i am in shanghai, china. the support tells me it problem is on my side (i guess the line from china to US). but what baffles me is that there is a large discrepancy between downloading from my droplet and downloading test files although sometimes test files are slow to download. but there is a period of time that i could download test file at speeds 100K-500K/s, and at the same time downloading from my droplet the speed is 10K-20K/s. i was told by support that the test files are hosted on the same kind of servers as my droplet and they should be representative of speed of my droplet. but here is a huge discrepancy. when i created my droplet, i did enable virtio. also i tried this experiment numerous times. when i download from my droplet, i was using nginx to download a plain file (by setting header), no extra processing at all. i also tried with downloading a file from filezilla, which does not go through nginx at all, just sshd. of course, this can be easily explained by my droplet sharing a port (or line) with other users who are using the line heavily, but the test file server does not. but i did this experiment many many times. if this is true all the time, then the test files are not representative of droplets. by the way, singapore dc is perhaps closest to where i am. but during the past week or so, most of the time, i had trouble downloading the test file from sg dc. only once it was fast and i created a droplet in singapore, it as only fine for a few minutes. then it slowed down to about 10K-20K/s. does anyone have experience in singapore dc? any idea is appreciated. thanks all in advanace.

dh