Sudden 5G Speed Drop from 300 Mbps to 40–50 Mbps – Strong Signal, No Congestion

[Limeyy]

Hi all,

I’m hoping someone here might be able to offer some insight, as I’ve been trying to resolve this for weeks without success.

Around 6–7 weeks ago my 5G home broadband speeds dropped overnight from a consistent 266–300 Mbps down to a stable 30–50 Mbps. The change was sudden and has remained completely consistent ever since.

Current setup:

• Three 5G Home Broadband

• Outdoor Greenpacket Y5-210MU router

• Approx. 440 metres direct line of sight to the mast

• Connected to 5G n78 (100 MHz)

• RSRP around -71 dBm

• SINR around 31 dB

• Carrier aggregation present

Internal network:

Originally:
Greenpacket → Eero 6 router → network

Now (for isolation/testing):
Greenpacket → Ubiquiti switch → Ubiquiti APs

I have also tested with the Greenpacket directly connected and isolated, removing other internal hardware from the equation.

Signal quality is excellent and unchanged.

Behaviour:

• Download ramps quickly to around 40–50 Mbps and then sits there

• Upload has increased from ~20 Mbps to ~30 Mbps

• No time-of-day variation (same speeds morning, evening, overnight)

• Same results across multiple speed test servers

• Same behaviour over wired and Wi-Fi

What I’ve done so far:

• Contacted Three support multiple times

• Router replaced (same SIM used initially)

• Recently obtained a brand new replacement SIM — no change

• Rebooted and power cycled multiple times

• Tested different internal network configurations

• Tried multiple speed test servers

• Confirmed strong signal and correct bands

The issue persists exactly the same across different routers, SIMs, and internal setups.

From testing and elimination this does not appear to be:

• Congestion (no time-of-day variation)

• Signal degradation (excellent RSRP and SINR)

• Router hardware fault

• Internal network bottleneck

The behaviour feels like a consistent throughput ceiling being applied upstream. The download speed hits ~40–50 Mbps quickly and stays there, rather than fluctuating like congestion would.

Has anyone experienced a similar sudden cap?
Could this be a mast-side configuration change or some form of network-level shaping?
Is there a specific way to get this escalated to the network team rather than standard troubleshooting?

Any advice would be greatly appreciated, as I feel I’ve exhausted the usual support routes.

Thanks in advance.

Spen

[J_Thompson]

did you get any help with this as my phone and Broadband have both dropped from 300mbp to 10mbs on a good day and i have done everything they suggest but nothing has worked also paid £85 to use my phone in Egypt and Greece and it didn't work after the first hour in Egypt and has no options for refunds this service is lacking in so many ways its unbelievable 

[Simonwaibw]

Traffic shaping / QoS profile

Mobile operators sometimes apply Quality of Service profiles to home broadband SIMs.

If a profile is mis-applied you can end up with a hard throughput ceiling like:

30 Mbps

50 Mbps

100 Mbps

The fact you briefly see 200–300 Mbps before it settles suggests the session starts unrestricted and then a policy rule kicks in.

2. Band fallback or anchor band limitation

5G NSA (which most UK mobile broadband uses) relies on a 4G anchor band.

If that anchor band is limited, your total throughput can look capped even though 5G is technically connected.

Typical LTE anchor limits can land right around:

40–60 Mbps

Which is suspiciously close to your numbers.

3. Backhaul capacity limit on the mast

Sometimes the cell sector backhaul gets capped or temporarily limited during upgrades or faults.

That can create a ceiling that affects home broadband devices first.

4. Device profile provisioning

Three’s network assigns devices a subscriber profile tied to the SIM IMEI class.

If the SIM has been placed in the wrong profile bucket you can see consistent ceilings.

Tests that help prove it

Engineers respond faster when the evidence is tidy.

Try:

1. Fast.com vs Speedtest

Netflix’s Fast.com often exposes shaping because it uses different traffic classification.

2. Multiple servers

Test to:

London

Manchester

Amsterdam

If every test hits the same ceiling, it strengthens the shaping theory.

3. Different device with the SIM

If possible:

put the SIM in a 5G phone

run a speed test outside

If the phone also caps around 40–50 Mbps, the issue is network-side, not router-side.

4. Check the router radio stats Look for:

RSRP

RSRQ

SINR

Band numbers

Good signal with a flat speed cap is another clue.