On my pc it used to read up to 70MB download but it only now reads 20MB download speed. The upload speed is the same 16MB. When someone came to my place and tried it on their mobile it still came up as 70MB download. It may be the processor in my computer as it is slow.

What is "it" in this context? A speed test?

Are you connected via WiFi or is your computer plugged into the router with a cable?

First though I had is that the OP is connected via WiFi and has dropped from 802.11N to 802.11G standard. That would certainly explain going from 70Mb to 20Mb. 20Mb is about what you'd expect for a 802.11G connection.