Vim 9 introduced new vim9script which improved performance by orders of magnitude from old vimscript.

You can test those benchmarks yourself from late Bramm's repo README file: https://github.com/brammool/vim9

My results are as follows:

  Indent time measurements

    888890                                                                                           
    Vim old:   0.678415                                                                              
    888890                                                                                           
    Python:   0.170969                                                                               
    888890                                                                                           
    Lua:   0.155279                                                                                  
    888890                                                                                           
    Vim new:   0.087310


    Sum time measurements:

    4499998500000                                                                                    
    Vim old:   6.286605                                                                              
    4499998500000                                                                                    
    Python:   0.439207                                                                               
    4499998500000                                                                                    
    Lua:   0.075501                                                                                  
    4499998500000                                                                                    
    Vim new:   0.119775

The Lua used here is LuaJIT, not standard Lua!