Watch for GPIO button while running a timer

Question

I've built to build a timer that shows the content in a 16x2 LCD.

So far so good. Now, what I want to achieve is that when I push a button, pause the timer.

I'm using ruby, with the library rpi_gpio and

This is my code:

require 'rpi_gpio'
require 'charlcd'

RPi::GPIO.set_numbering :bcm

RPi::GPIO.setup 18, :as => :input, :pull => :up

lcd = CharLcd.new
lcd.begin(16, 2)
lcd.clear

total_seconds = 0
start_time = Time.now

t = Thread.new do
    loop do
        sleep 1
        total_seconds = Time.now - start_time
        hours = (total_seconds/ 3600).to_i
        minutes = ((total_seconds % 3600) / 60).to_i
        seconds = ((total_seconds % 3600) % 60).to_i

        lcd.clear
        tim =  hours.to_s.rjust(2, '0') + ':' + minutes.to_s.rjust(2, '0') + ':' + seconds.to_s.rjust(2, '0')
        lcd.message(tim) 
    end
end

while RPi::GPIO.high? 18 do
end
t.kill

lcd.clean_pins()

The problem comes when, as far as I know, the only way to control when button is pushed is with a loop, and I think two loops (the one of the timer and the one that controls the button) make slow the printing of the message in the LCD display.

I'm record this video to show you guys the behavior:

https://youtu.be/4P2OsyGr0D4

I'd be grateful if somebody could help me out with this, or give me another solution.

Thanks in advanced.