Are you saying you can read the OP's mind? Because
"I have observed a "slowness" to my general internet experience lately"
is a very vague statement. So please do share precisely what he/she means by "slowness" because the OP didn't expand on that.
What the OP did make clear was, they wanted to know why the TBB results were lower than Ookla's. So based on my experience and some other posters - such as those with Vodafone & Virgin Media - I suggested the TBB tester doesn't always give out the correct results, for whatever reason.
Does that make sense now?
BT Business FTTP(oD) 330/50 -- 4+2 ONT -- Netgear RAX200