Does running a full node make rpc calls faster?
This has nothing to do with pruning.  RPC calls like getbalance only use data stored in the wallet and/or UTXO database.  They don't have any need to refer to the archive of old blocks, and having it available wouldn't help anything.

VPSs often don't have a lot of RAM, and bitcoind in its default configuration needs a lot - say a gigabyte or more.  Is it possible that you're short on physical memory and therefore having to swap?  This can make a system very slow.  You can reduce bitcoind's memory usage somewhat with different options (see How to run bitcoind in a low memory environment) but you may also have to configure your VPS with more memory (and pay more).

Another possibility is that your server is overloaded with other tasks.

Either way, running top will probably reveal something.