there are a lot of factors that will impact your download speed and although the Speed Test is a useful guide it is only a snapshot of your speed from one particular site.

For instance:
I run a (hypothetical) website from my home using my current ISP.
My Download speed, according to Speed Test is 26Mbps
My Upload speed is 3Mbps

You come along and access my site and download a file from it.
You cannot pull that file off my server faster than I can upload it so your 5.24Mbps download speed gets to be significantly less.

That said, when you are slowing down, have you gone back and done a speed test? If so, what were the results?

Emac's were not the fastest horse in the stable, even when they were new, so old age and over heating may be a problem here too.
Check the innards for dust bunnies and the like