Work mate's (@LukeGumbley) suggestion - read log files in compressed, very useful for large log files - big data stuff. The reduction in IO makes parsing lines substantially quicker. The test.zip file used for the code below held 100 log files totalling 500MB raw and 16MB zipped and all lines were processed in a just a few seconds on a laptop.

zipLogReader.fsx

#r "System.IO"
#r "System.IO.Compression"
#r "System.IO.Compression.FileSystem"

open System.IO
open System.IO.Compression

// open up streams from there
for entry in Compression.ZipFile.OpenRead(@"c:/temp/test.zip").Entries do
    printfn "%s" entry.FullName
  
    let logLines = seq {
         
        use reader = new StreamReader (entry.Open() )
        while not reader.EndOfStream do
            yield reader.ReadLine()
    }

    printfn "%i" (logLines |> Seq.length)