operations: disable multi thread copy for local to local copies

...unless --multi-thread-streams has been set explicitly

docs/content/docs.md

@@ -743,6 +743,10 @@ above.
 **NB** that this **only** works for a local destination but will work
 with any source.
 
+**NB** that multi thread copies are disabled for local to local copies
+as they are faster without unless `--multi-thread-streams` is set
+explicitly.
+
 ### --multi-thread-streams=N ###
 
 When using multi thread downloads (see above `--multi-thread-cutoff`)

fs/config.go

@@ -100,6 +100,7 @@ type ConfigInfo struct {
 	ClientKey              string // Client Side Key
 	MultiThreadCutoff      SizeSuffix
 	MultiThreadStreams     int
+	MultiThreadSet         bool // whether MultiThreadStreams was set (set in fs/config/configflags)
 }
 
 // NewConfig creates a new config with everything set to the default

fs/config/configflags/configflags.go

@@ -213,4 +213,9 @@ func SetFlags() {
 	if err == nil {
 		config.ConfigPath = configPath
 	}
+
+	// Set whether multi-thread-streams was set
+	multiThreadStreamsFlag := pflag.Lookup("multi-thread-streams")
+	fs.Config.MultiThreadSet = multiThreadStreamsFlag != nil && multiThreadStreamsFlag.Changed
+
 }

fs/operations/operations.go

@@ -310,7 +310,10 @@ func Copy(ctx context.Context, f fs.Fs, dst fs.Object, remote string, src fs.Obj
 		}
 		// If can't server side copy, do it manually
 		if err == fs.ErrorCantCopy {
-			if doOpenWriterAt := f.Features().OpenWriterAt; doOpenWriterAt != nil && src.Size() >= int64(fs.Config.MultiThreadCutoff) && fs.Config.MultiThreadStreams > 1 {
+			// Disable multithread if --multi-thread-streams not in use and source is local
+			disableMultiThread := !fs.Config.MultiThreadSet && src.Fs().Features().OpenWriterAt != nil
+			doOpenWriterAt := f.Features().OpenWriterAt
+			if !disableMultiThread && doOpenWriterAt != nil && src.Size() >= int64(fs.Config.MultiThreadCutoff) && fs.Config.MultiThreadStreams > 1 {
 				// Number of streams proportional to size
 				streams := src.Size() / int64(fs.Config.MultiThreadCutoff)
 				// With maximum