The only fast part in Roundcube seems to be loading the sendmail screen. TinyMCE will trash that loading time. Please keep the editor the way it is (why the f would you need all those bullshit options anyway) or at least make the bloatware optional.
( ) Default Roundcube composer
( ) Active bloatware TinyMCE
Nobody has been forced to use it, it's always been an option.