63*1024*100 is way too much... this is about 6MB per connection...
Raising buffer sizes will always also raise memory use. I would not recommend you values above 512*1024.

When raising the buffers you also need to raise your 'Initial download segment size' and the 'Target time per segment', since large buffers dont help you when the requested download segments are much smaller then the buffers.

Phex is almost not using flush() during downloads.

Gregor

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