Just checked out the beta myself -- I notice the same performance problems, including the lengthy pause after each download completes, but turning off the new "automatically share downloaded files" and always manually deciding what to share seems to fix that one. There's a couple weird/funky bugs though.

1. The green check symbol most definitely doesn't work as intended. I saw one I was sure I didn't have and went to download it -- nothing. No file overwrite prompt, nor the new "the file matches content in your library" one. There's maybe one file like this per search, given a bogus green check. So grabbing everything in some group of files still means selecting them all, regardless of green check, hitting "download", and wading through the resulting overwrite prompts. Otherwise you risk missing something. Sigh.

2. Downloads sometimes get stuck at 100%. Once in a while a download finishes but doesn't go to "Complete" status (and then, if auto-clear is on, disappear) -- instead it stays saying "downloading", 100% progress, 0kb/s throughput. Hitting "pause" and then "resume" results in the file being stuck "queued" with 100% progress, rather than it doing whatever it's supposed to do and granting it "complete" status.

3. It seems to be much harder to get files from a "reluctant" host. You know the type -- you find some interesting files in a group that prove to be co-hosted; select them all; and three of them download and the rest say "Need more sources". Hit "find sources" on these, and three more download and the rest say "Awaiting sources". Previously (4.8.1 and earlier) you could select these and hit resume, and you'd get another three files, and so forth, eventually getting them all. In the 4.9.0 beta, this doesn't work -- the files will go to "queued" very briefly, and then back to "awaiting sources", or even fail to change status at all. Apparently if a host does the wrong thing and pretends to have gone offline in lieu of sending a civilised busy signal, 4.9.0 will forget it ever existed. Repeating the search that found the files will usually (but by no means always) cause the next three to download. It should be noted that some of the 4.x Limewires are among the hosts with the bad busy-signal behavior described above, where they act like they've gone offline rather than are merely busy when the max concurrent downloads per downloader is reached.