I created two test projects. One that does AssemblyInfo Updater and one that does Text Replace on the 4th digit (RevisionVersion) on 350 AssemblyInfo.cs files. The Run Times are:
Would you be able to send your test projects to support. There are a number of options on each of the actions and I have found having sample projects is the quickest means to convey the circumstance where the issues exists.
Just taking a quick look at the code, assemblyInfo Updater will always be quicker it knows the format of the file. So it just updates the structure then writes it back, updating the parts of the assembly info which require it. The Text Find / Replace on the other hand has to review the entire string each time, locate what requires updating, the alter it. Needless to say I wouldn’t have thought it was that slow, so please send in your example projects and I will look at improving its speed.