user139009: I'm running a full bitcoin node. No pruning. Two primary use cases:

Parse the most recent block and all the transactions in it
Fetch any given address balance

Both these can be achieved by just the bitcoind service and using its RPC methods. Coming to the question, why has the community built electrumx, electrs on top of it? I read they make some queries faster. But none of them explain which queries and how much of an efficiency improvement over just running bitcoid.

user48884: 
Fetch any given address balance

This cannot be done with Bitcoin Core. ElectrumX and electrs are examples of software that maintain an address index, which is a database that allows the lookup of the transactions, balance, UTXOs, etc. of any address. Bitcoin Core does not have an address index, and so it cannot be used for any task which requires fast lookup of this information.
You may see RPCs in Bitcoin Core which seem like they would allow for the arbitrary address information lookup. However these are often wallet RPCs and can only do these lookups for anything that the loaded wallets own, not any arbitrary address. There are also some RPCs that can compute this information as needed, but these end up being very slow and so unusable for the tasks that external address indexes are often used for.

139009: <p>I'm running a full bitcoin node. No pruning. Two primary use cases:</p>
<ul>
<li>Parse the most recent block and all the transactions in it</li>
<li>Fetch any given address balance</li>
</ul>
<p>Both these can be achieved by just the <code>bitcoind</code> service and using its RPC methods. Coming to the question, why has the community built <code>electrumx</code>, <code>electrs</code> on top of it? I read they make some queries faster. But none of them explain which queries and how much of an efficiency improvement over just running <code>bitcoid</code>.</p>
 48884: <blockquote>
<p>Fetch any given address balance</p>
</blockquote>
<p>This cannot be done with Bitcoin Core. ElectrumX and electrs are examples of software that maintain an address index, which is a database that allows the lookup of the transactions, balance, UTXOs, etc. of any address. Bitcoin Core does not have an address index, and so it cannot be used for any task which requires fast lookup of this information.</p>
<p>You may see RPCs in Bitcoin Core which seem like they would allow for the arbitrary address information lookup. However these are often wallet RPCs and can only do these lookups for anything that the loaded wallets own, not any arbitrary address. There are also some RPCs that can compute this information as needed, but these end up being very slow and so unusable for the tasks that external address indexes are often used for.</p>
