1 reason, there's a lot of players on the server sometimes. Especially on weekends reason 2, it could just be your laptop being laggy. Mine is especially laggy on Minecraft, so I'm not sure ask someone who know's more stuff about lag than I do because I'm basically just a minecraft noob
It could also be your wifi, as mine tends to make Minecraft laggy at certain times of the day. I know it is the wifi because my computer is relatively new, with good processing and works fine 80% of the time when the wifi isn't acting up
Minecraft is not a well optimized game. It runs great on some computers and terrible on others. Installing Optifine should help quite a bit though.
Also, if you have a Mac those computers aren't really made for playing games on. So it would be harder for those to load everything happening in the game. (Especially on a server as big as sandlot) I have noticed the server lags a lot between 6pm and 7:30pm on different days.
