Hello Guys,
it seems this is really caused by the nature of the WebShield implementation. The super-fast upload phase is caused by the fact, the until avast buffers are filled, the data is consumed by WebShield, scanned and then passed onto the network. There the speed should be appropriate to your network speed, however since the application (dropbox) has already sent quite a lot of bytes, it would seem as stalled for a while. In total, the speed shouldn't be affected, the total time should be the same, however you could see these super-fast bursts. We haven't experienced any complete stalls or aborted uploads though. We will look into this and post you soon.
Sorry guys,
Lukas.