Pro and Go are very different from WebDirect. On the whole what makes a difference is how much data you are requesting from the server. Pro and Go caches layouts, scripts and previously viewed records so that after the first visit or view it is only changed or fresh data that needs to be delivered.
Put container data such as files into their own table. If you are storing pictures make sure they are not bigger than needed. Once you know the maximum size you need you can set this into a field definition Auto enter calculation which can replace. From that point on all pictures that are put into that field will be reduced in size. For instance: GetThumbnail ( Picture ; 640 ; 640 ) ensures no pictures stored are ever wider or taller than 640 pixels.
Summary fields have a large impact. Do they need to be displayed all the time? If not consider duplicating the layout removing the summary fields from the original and provide a button to switch between the two versions.
Calculation fields make tables ‘wider’ often for no reason. With FMP 14 you can now do calculations on your layout using an empty global field displayed with calculated inline text. eg if you have first name and last name you may no longer need a calculation that joins them.