I used to smoke a couple hours before work, but I got way too paranoid that someone was going to find out (my job has a super strict anti drug policy and will test you if they think you are acting 'off'.) When I first started, I only felt a little stoned when I came to work and I loved the relaxed feeling, but I have a ton of customers to deal with and its a very fast paced environment so it was hard for me to deal with the customers as efficiently.
Around a year ago I smoked waay too much before work and I know I was acting really strange and I found it super difficult to comprehend any directions. When my boss walked up to me I was 100% sure he was going to pull me aside and tell me he smelled weed on me and that I was going to have to take a drug test, BUT he just ended up walking by and not saying a word. After that I have only smoked before work a couple times b/c I had a really bad headache and by the time I got to work I was only feeling a slight buzz. Now I wait until after work.