Based on average read speed, do you use USB 2 (whether the port is USB 2 or the external HDD uses USB 2) ?
I think it's USB 3 (the external HDD is the Seagate device - bus 004)
Code:
>lsusb
Bus 002 Device 002: ID 8087:8000 Intel Corp.
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 8087:8008 Intel Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 007: ID 0bc2:231a Seagate RSS LLC
Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 003 Device 003: ID 138a:0017 Validity Sensors, Inc. Fingerprint Reader
Bus 003 Device 010: ID 0458:0186 KYE Systems Corp. (Mouse Systems)
Bus 003 Device 002: ID 1058:10b8 Western Digital Technologies, Inc. Elements Portable (WDBU6Y, WDBUZG)
Bus 003 Device 016: ID 1b3f:2008 Generalplus Technology Inc.
Bus 003 Device 005: ID 04ca:7035 Lite-On Technology Corp.
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 002: ID 8087:8000 Intel Corp.
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 8087:8008 Intel Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 007: ID 0bc2:231a Seagate RSS LLC
Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 003 Device 003: ID 138a:0017 Validity Sensors, Inc. Fingerprint Reader
Bus 003 Device 010: ID 0458:0186 KYE Systems Corp. (Mouse Systems)
Bus 003 Device 002: ID 1058:10b8 Western Digital Technologies, Inc. Elements Portable (WDBU6Y, WDBUZG)
Bus 003 Device 016: ID 1b3f:2008 Generalplus Technology Inc.
Bus 003 Device 005: ID 04ca:7035 Lite-On Technology Corp.
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
May not get any answers due to the ElectrumX dev committing to BSV, but have you tried logging an issue on the ElectrumX GitHub? 
(or maybe the fork of ElectrumX that is still supporting BTC)
(or maybe the fork of ElectrumX that is still supporting BTC)I know... I guess electrs will take the lead But last time I checked electrs was very bad at handling mempool transactions... any other electrum server implementation besides electrumX or electrs?