Compression test:
127 506 files (a svn wc) Original size: 3 443 130 254
zlibnet
compressed (default): 1,09 GB (1 178 180 638 bytes) time: 00:02:39.9731499
.NET 4.5 ZipArchive
compressed (optimal): 1,08 GB (1 168 228 598 bytes) time: 00:03:00.2553101
compressed (fastest): 1,18 GB (1 270 258 799 bytes) time: 00:02:00.8949148
ZipArchive performace is actually not that bad:-)