It appears that the version number of this assembly is locked at 6.0.0.0. This can be problematic for VBCSCompiler during reference package updates, leading to dramatic reduction in compiler throughput (for Roslyn.sln, builds are about 3x slower). The fix for this solution is ensuring the assembly version number increases with each build for analyzer and source generator packages.
Originally posted by @sharwell in #58175 (comment)