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Bumping this since I'm facing the same issue. I've been experiencing this since late October 2020.
I just had a downtime for a night and in the morning I wanted to check my internet connection, since I haven't checked that in a few months. I was quite surprised by the results: my speed was between 46 and 50 Mbits whilst the minimum guaranteed is 58Mbits.
I decided to get in touch with the support and they sent me an engineer. He came in, checked the wires outside and inside my flat and said that everything was ok from his side. He just told me that he "reset my profile" and in about a week I should have my guaranteed speed back. He suspected that my modem kept disconnecting, therefore the speed got slower to remove noise from the line.
I wasn't too worried, since 48MBits are quite enough to watch Netflix, play games, work, make video-calls and download large files. Last week I started streaming my gameplays on Facebook (after 3 months) and I noticed lags and stuttering in both the live stream and the gameplay. I got in touch with BT's customer support and they sent me a new BT Hub. I got it today and I was so happy that a I got my speed closer (55MBits) to my guaranteed one, however I noticed huge lag spikes happening about every 5 minutes that made online gaming unplayable (I've exported a clip to YouTube, so you can see the effects of a 900+ms ping).
I've checked all my cables, my equipment, and I've also connected directly my PC to the router, but still, I got huge spikes. Here are other evidences using PingPlotter, Ping and a speed test.