Just upgraded to M500 from M350 and despite everything being the same set up wise I'm experiencing very inconsistent speeds (dropping to around 40mb) often and much slower over affecting gaming etc. This was never the case on M350 we consistently had good speeds.
Can you set up a Broadband Quality Monitor (BQM) at thinkbroadband.com - this will give you an insight into what is happening with the signal at the other side of the Hub, it will take a few hours to get any kind of trend showing although you should post the link straight away.
Post a 'Live Link' to your BQM on here.
Instructions for posting BQM Link
Under your BQM graph are two links in red.
Click the lower link (Share Live Graph) then click generate.
Copy the text in the Direct Link box, beware, there may be more text than you can see.
On here click the Link icon (2 links chain to the left of the camera icon)
In the URL box paste the link you copied and in the ‘text to display’ box write My BQM then click OK - you can post the link straight away.
Type 192.168.0.1 (192.168.100.1 in Modem mode) into your browser URL bar and press enter.
Hub 2 & 3 When the page appears DO NOT LOG IN but click ‘Check Router Status’.
Hub 4 When the page appears LOG IN then Advanced Settings > Tools > Network Status.
Copy and paste the contents of the Downstream, Upstream, Configuration and Network Log tabs onto here, if you get a yellow / straw box warning click the Post button again. Use one post for each tab if you wish.
Glad I found this topic as I've just experienced a similar thing. Received a marketing call on 26 or 27 November about some benefits VM could offer me to my package. Whilst I originally declined the £16 increase I folded when they reduced the rise to about £12 as I was really too busy to haggle at the time. Included with the sky sports upgrade (which they didn't mention only gives you the non-HD SKY channels when I'm already on a premium tv package) was a speed increase from 350 to 500Mbps.
Having changed nothing on my network except a restart of my Hub 3 when speed appeared to decline, I am left with an average Ookla result in the mid-200s depending on the time of day. Not sure time of day matters as it's totally random when speed drops or increases occur. One of my wired laptops, used to nudge into the 300Mb range more often than not whilst on M350 but has never achieved 260Mb since and mostly gets 180-220ish. An iPhone XS could previously manage up to around 360Mb but has never been close to even 300Mb since the 'upgrade' to M500. I really wish I'd persisted in declining the upgrade because now I'm paying £12 more for less every month. Thought I'd hit the jackpot in finding a solution in this thread and expected to find my power levels low as detailed here, but across the 24 bonded channels, they range between +1.5 and -2.0 so I'm stumped. Half expecting VM to tell me that unprecedented network traffic over the Black Friday weekend is the cause but that would just send me over the edge. I've tested at 3-4am when most people are in bed but still the same so can't be number of active users.
VM - how do we fix this otherwise I want to cancel my 'upgrade'?