Just wanted to provide a quick update. I finally bought the slick tires yesterday. I installed them myself as it wasn't too challenging. You were right Jim I tried them out this morning and the difference was noticeable. I wouldn't say I was going much faster but the ride was definitely more comfortable and fun. The guy at the bike shop recommended that I don't spend money on this bike as it's too small for me (24") and that I should rather just buy a 26" bike. But I think I am going to save my money and buy a better bike next summer. For now, I am doing about 100 km a week and I want to increase that gradually and also try to include more hill work.

Thanks to everyone for their help!