fsharp-dotnet, 140 bytes

Result

AC

Duration

7550ms

Code [DL]

let rec f x=let c=stdin.Read()in let x=(match c with-1->exit 0|84->0|75->x-50|_->x+1)%51 in stdout.Write(if x<1 then 'x'else char c);f x
f 0

stdin

                                    T             
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
       K                                          

stdout

                                    x             
                                    x             
                                    x             
                                    x             
                                    x             
                                    x             
                                    x             
                                    x             
                                    x             
       xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx             

stderr

The template "Console Application" was created successfully.

Processing post-creation actions...
Running 'dotnet restore' on /tmp/Proj/Proj.fsproj...
  Restoring packages for /tmp/Proj/Proj.fsproj...
  Generating MSBuild file /tmp/Proj/obj/Proj.fsproj.nuget.g.props.
  Generating MSBuild file /tmp/Proj/obj/Proj.fsproj.nuget.g.targets.
  Restore completed in 216.9 ms for /tmp/Proj/Proj.fsproj.

Restore succeeded.

strace