TL;DR
Local instances of BTC RPC Explorer and mempool.space work nicely with Fulcrum.

I'll have to dig out how it's configured as I've used it with RaspiBlitz where you simply install it (ok, switching from electrs to Fulcrum needs a tweak or two in earlier versions of RaspiBlitz).

I've used Fulcrum in my RaspiBlitz node setup instead of electrs and had local instances of blockchain explorers like BTC RPC Explorer aka bitcoinexplorer.org and mempool.space. It has worked like a charm until I ran out of space on the 1TB storage media (Fulcrum takes considerably more space than electrs).

From my own experience electrs is significantly slower for large address histories than Fulcrum and this was for me the main driver to switch from electrs to Fulcrum.

Why am I talking of past stuff? Well, I've been a bit lazy to upgrade my RaspiBlitz node and setup a more recent version of it. I also wanted to see how RaspiBlitz evolves (I started with v1.7.0 upgraded a few versions from there and tweaked it here and there to my liking... until it broke.)

As far as I can tell from syncing my Fulcrum: it took a while and syncing it is more time consuming than with electrs. But my Bitcoin Core was in sync back then. Fulcrum won't definitely respond to queries as long as it isn't fully synced and as long as the Bitcoin Core it talks to isn't synced either.



The issue might not be your internet connection but additionally maybe too small value for dbcache due to not enough free RAM available. On what kind of storage media is your blocks and chainstate directories located, hopefully a SSD?