c-gcc, 135 bytes

Result

AC

Duration

877ms

Code [DL]

main(d,c,b){
	for(;c-10;){
		c=getchar();
		if(c>=65){
			d=c; putchar(c);
		}
		else{
			for(;c>49;c--){
				putchar(d);
			}
		}
	}
}

stdin

c8e3r6q3t7a2t4q7gj9u5c6l9v5p2q9b2jo2fp4z9jb9mdxb7n9w2k2j8ci5x2g3r5p6f4g3k6d6n5s8yb7h5r3n6x5

stdout

cccccccceeerrrrrrqqqtttttttaattttqqqqqqqgjjjjjjjjjuuuuucccccclllllllllvvvvvppqqqqqqqqqbbjoofppppzzzzzzzzzjbbbbbbbbbmdxbbbbbbbnnnnnnnnnwwkkjjjjjjjjciiiiixxgggrrrrrppppppffffgggkkkkkkddddddnnnnnssssssssybbbbbbbhhhhhrrrnnnnnnxxxxx

stderr

/tmp/code.c:1:1: warning: return type defaults to 'int' [-Wimplicit-int]
 main(d,c,b){
 ^~~~
/tmp/code.c: In function 'main':
/tmp/code.c:1:1: warning: type of 'd' defaults to 'int' [-Wimplicit-int]
/tmp/code.c:1:1: warning: type of 'c' defaults to 'int' [-Wimplicit-int]
/tmp/code.c:1:1: warning: type of 'b' defaults to 'int' [-Wimplicit-int]
/tmp/code.c:3:5: warning: implicit declaration of function 'getchar' [-Wimplicit-function-declaration]
   c=getchar();
     ^~~~~~~
/tmp/code.c:3:5: note: 'getchar' is defined in header '<stdio.h>'; did you forget to '#include <stdio.h>'?
/tmp/code.c:1:1:
+#include <stdio.h>
 main(d,c,b){
/tmp/code.c:3:5:
   c=getchar();
     ^~~~~~~
/tmp/code.c:5:9: warning: implicit declaration of function 'putchar' [-Wimplicit-function-declaration]
    d=c; putchar(c);
         ^~~~~~~

strace