Learning assembly. how to make code faster - string

Question

I started to learn assembly some days ago and i write my first ever piece of code using user input, string functions, passing arguments by stack or by register etc...
I have some questions. Do you have some advices to make my code faster. For example, in my atoi function, i know that imul is time consuming. Maybe, there are enormous mistakes but as far as i know, many things to improve for sure. So my main question is : are there fatal errors in this first code and my second is : any type to refactoring code with faster instructions
SYS_READ equ 3
SYS_WRITE equ 4
STDIN equ 0
STDOUT equ 1
%macro printm 2
mov eax, SYS_WRITE
mov ebx, STDOUT
mov ecx, %1
mov edx, %2
int 0x80
%endmacro
%macro prolog 0
push ebp,
mov ebp, esp
%endmacro
%macro epilog 0
mov esp, ebp
pop ebp
%endmacro
section .text
global _start
_start:
; first check if our strlen proc works
push dword msgbegin
call strlen
add esp, byte 4
cmp eax, lenbegin
je .suite ; it works, we continue
; exiting prog if the len computed in rax != lenbegin
mov eax, 1
int 0x80
.suite:
; check if strcpy works printing res (msgbegin -> srcdst)
push dword lenbegin
push dword msgbegin
push dword strdst
call strcpy
add esp, byte 12
push dword lenbegin
push dword strdst
call print
add esp, byte 8
; first input
printm msgbinp1, leninp1
mov eax, SYS_READ
mov ebx, STDIN
mov ecx, num1
mov edx, 2
int 0x80
printm msgbinp2, leninp2
mov eax, SYS_READ
mov ebx, STDIN
mov ecx, num2
mov edx, 2
int 0x80
printm msgbinp3, leninp3
mov eax, SYS_READ
mov ebx, STDIN
mov ecx, bignum
mov edx, 4
int 0x80
mov edx, bignum
call atoi
cmp eax, 123
je .success ; exit if bignum != 123
mov eax, 1
int 0x80
.success:
; need to strip line feed from bignum
printm bignum, 4
printm msgoutp, lenoutp
; now we compute the sum
mov eax, [num1]
sub eax, '0'
mov ebx, [num2]
sub ebx, '0'
add eax, ebx
add eax, '0'
mov [sum], eax
printm msgres, lenres
; we print it
printm sum, 1
; exiting the programm
mov eax, 1
int 0x80
print:
push ebp
mov ebp, esp
mov eax, 4
mov ebx, 1
mov ecx, [ebp + 8]
mov edx, [ebp + 12]
int 0x80
mov esp, ebp
pop ebp
ret
strcpy:
push ebp
mov ebp, esp
mov ecx, [ebp + 16]
mov esi, [ebp + 12]
mov edi, [ebp + 8]
rep movsb
mov esp, ebp
pop ebp
ret
strlen:
push ebp
mov ebp, esp
push edi
push ecx
mov edi, [ebp + 8]
sub ecx, ecx
sub al, al
not ecx
cld
repne scasb
not ecx
lea eax, [ecx] ; keep null term in size
pop ecx
pop edi
mov esp, ebp
pop ebp
ret
atoi:
xor eax, eax ; zero a "result so far"
.top:
movzx ecx, byte [edx] ; get a character
inc edx ; ready for next one
cmp ecx, '0' ; valid?
jb .done
cmp ecx, '9'
ja .done
sub ecx, '0' ; "convert" character to number
imul eax, 10 ; multiply "result so far" by ten
add eax, ecx ; add in current digit
jmp .top ; until done
.done:
ret
section .data
msgbegin db "hello everyone !", 0xa, 0
lenbegin equ $ - msgbegin
msgbinp1 db "Enter a digit : ", 0xa, 0
leninp1 equ $ - msgbinp1
msgbinp2 db "Enter second digit : ", 0xa, 0
leninp2 equ $ - msgbinp2
msgbinp3 db "Enter third digit : ", 0xa, 0
leninp3 equ $ - msgbinp3
msgoutp db "is equal to 123 !", 0xa, 0
lenoutp equ $ - msgoutp
msgres db "sum of x and y is ", 0xa, 0
lenres equ $ - msgres
strdst times lenbegin db 0
segment .bss
sum resb 1
num1 resb 2
num2 resb 2
bignum resd 4
Thanks you. I started reading the doc but i'm not sure that i understood key concepts.