Shiny + SQLite - why is Shiny extremely slow?

We have developing a Shiny app for a few months now. But our Shiny app is extremely slow when it tries to load a huge amount of data. We even use the reactive function to reuse the data. But it is still slow as before when we request different sets of data.

We have a log file and it shows that Shiny takes at least 30.12672 seconds or 52.24799 seconds each time to load the data from our database.

What are the reasons make Shiny so slow? Is it the server or the database? What can we do to speed it up?

We are using SQLite database. Is it the reason that makes Shiny slow?

If so, what other types of database system should we go for to process huge amount of data sets? Cassandra? HBase? Apache Spark?