the vehicle's starter is slow

Until recently, the vehicle started to take one starter and was working without problems. Then, when I turned the ignition, it started to work with a little delay, not one time. he took the starter alone and grasped it in 3-5 seconds. 2 days ago, I had to work hard. the same thing happened this morning. I said I'll change the battery, I got the battery measured. I went to the electrician, hit two contacts, the starter does not have a good problem, look at the battery. I am in between now, now I do not get what I want to buy or remove the battery. friends with advice or experience I am waiting for your help.

how much fuel is there in the tank If it is at the very bottom, it takes a little late. If it is diesel outside, glow plugs, the starter engine comes to my mind first.