Compression speed is subjective to the type of content being compressed. Unity uses the Lz4 source directly for asset bundle compression, specifically using the Lz4HC variant at maximum compression levels which is slower to compress than standard Lz4, but achieves better file sizes while keeping the same fast Decompression speeds. In the case of these textures, the issue is the Lz4 source itself when using compression levels over the default level for Lz4HC. The Scriptable build pipeline (2018.2+) exposes the BuildCompression options used and they can set the compression level for Lz4HC to default (CompressionLevel.High) if they don't mind the extra file size. I would avoid changing the compression block size if an attempt to improve compression size or speed as this greatly impacts Decompression speeds.